Hello,
I have a very strange problem: Since I have my new PC, I have always had problems with it. From time to time the computer freezes for some seconds and suddendly reboots by itself. I've had this problem since Ubuntu 9.10. The same with 10.04 and 10.10. That's why I don't think it's a software failure because the problem persist too long. It doesn't have anything to do with what I'm doing at this time. Sometimes I listen to music, sometimes I only use Firefox, sometimes I'm running 2 or 3 VMs, sometimes I watch DVD. So it's not isolatable. I could freeze once a day or once a week. I put the PC to the vendor twice(!). The first time they changed my power supply but the problem persisted. The second time they told me that they made some heavy performance tests 50 hours long but they didn't find anything. (How can that be that I have daily freezes with normal usage). The vendor didn't check the hard discs because they used their own disc with Windows. (So they never checked the Linux installation).
Yesterday I made some intensive hard disc scans with "SMART" but no errors were found. I ran memtest for 3 times but no errors found. I already had this problem in my old flat, so I doubt that I has something to do with current fluctuation. I already tried another electrical socket and changed to connector strip but the problem persists. At the moment I removed 2 of the RAMs (2x 2GB). In all I have 6GB, 2x2GB and 2x1GB. Could this difference maybe be a problem? Here is a list of my components. I hope that anybody find something I didn't think about yet.

And here a list of my components:

1x AMD Phenom II X4 965 Black Edition, 3,4Ghz, Quad Core, S-AM3, Boxed
2x DDR3-RAM 2048MB, PC3-1333 Mhz, CL9, Kingston ValueRAM
2x DDR3-RAM 1024MB, PC3-1333 Mhz, CL9, Kingston ValueRAM
2x SATA II Seagate Barracuda 7200.12, 1TB 32MB Cache => RAID 1
1x DVD ROM SATA LG DH16NSR, 16x/52x
1x DVD-+R/-+RW SATA LG GH-22NS50
1x Cardreader 18in1
1x PCI-E 2.0 GeForce GTS 250, Retail, 1024MB
1x Power Supply ATX 400 Watt, CHIEFTEC APS-400S, 80 Plus
1x Network card PCI Intel PRO/1000GT 10/100/1000 MBit
1x Mainboard Socket-AM3 ASUS M4A79XTD EVO, ATX

lshw:

    description: Desktop Computer
    product: System Product Name
    vendor: System manufacturer
    version: System Version
    serial: System Serial Number
    width: 64 bits
    capabilities: smbios-2.5 dmi-2.5 vsyscall64 vsyscall32
    configuration: boot=normal chassis=desktop uuid=80E4001E-8C00-002C-AA59-E0CB4EBAC29A
  *-core
       description: Motherboard
       product: M4A79XTD EVO
       vendor: ASUSTeK Computer INC.
       physical id: 0
       version: Rev X.0X
       serial: MT709CK11101196
       slot: To Be Filled By O.E.M.
     *-firmware
          description: BIOS
          vendor: American Megatrends Inc.
          physical id: 0
          version: 0704 (11/25/2009)
          size: 64KiB
          capacity: 960KiB
          capabilities: isa pci pnp apm upgrade shadowing escd cdboot bootselect socketedrom edd int13floppy1200 int13floppy720 int13floppy2880 int5printscreen int9keyboard int14serial int17printer int10video acpi usb ls120boot zipboot biosbootspecification
     *-cpu
          description: CPU
          product: AMD Phenom(tm) II X4 965 Processor
          vendor: Advanced Micro Devices [AMD]
          physical id: 4
          bus info: cpu@0
          version: AMD Phenom(tm) II X4 965 Processor
          serial: To Be Filled By O.E.M.
          slot: AM3
          size: 800MHz
          capacity: 3400MHz
          width: 64 bits
          clock: 200MHz
          capabilities: fpu fpu_exception wp vme de pse tsc msr pae mce cx8 apic sep mtrr pge mca cmov pat pse36 clflush mmx fxsr sse sse2 ht syscall nx mmxext fxsr_opt pdpe1gb rdtscp x86-64 3dnowext 3dnow constant_tsc rep_good nonstop_tsc extd_apicid pni monitor cx16 popcnt lahf_lm cmp_legacy svm extapic cr8_legacy abm sse4a misalignsse 3dnowprefetch osvw ibs skinit wdt npt lbrv svm_lock nrip_save cpufreq
        *-cache:0
             description: L1 cache
             physical id: 5
             slot: L1-Cache
             size: 512KiB
             capacity: 512KiB
             capabilities: pipeline-burst internal varies data
        *-cache:1
             description: L2 cache
             physical id: 6
             slot: L2-Cache
             size: 2MiB
             capacity: 2MiB
             capabilities: pipeline-burst internal varies unified
        *-cache:2
             description: L3 cache
             physical id: 7
             slot: L3-Cache
             size: 6MiB
             capacity: 6MiB
             capabilities: pipeline-burst internal varies unified
     *-memory
          description: System Memory
          physical id: 36
          slot: System board or motherboard
          size: 2GiB
        *-bank:0
             description: DIMM Synchronous 1333 MHz (0.8 ns)
             product: ModulePartNumber00
             vendor: Manufacturer00
             physical id: 0
             serial: SerNum00
             slot: DIMM0
             size: 1GiB
             width: 64 bits
             clock: 1333MHz (0.8ns)
        *-bank:1
             description: DIMM Synchronous 1333 MHz (0.8 ns)
             product: ModulePartNumber01
             vendor: Manufacturer01
             physical id: 1
             serial: SerNum01
             slot: DIMM1
             size: 1GiB
             width: 64 bits
             clock: 1333MHz (0.8ns)
        *-bank:2
             description: DIMM [empty]
             product: ModulePartNumber02
             vendor: Manufacturer02
             physical id: 2
             serial: SerNum02
             slot: DIMM2
        *-bank:3
             description: DIMM [empty]
             product: ModulePartNumber03
             vendor: Manufacturer03
             physical id: 3
             serial: SerNum03
             slot: DIMM3
     *-pci:0
          description: Host bridge
          product: RD780 Northbridge only dual slot PCI-e_GFX and HT1 K8 part
          vendor: ATI Technologies Inc
          physical id: 100
          bus info: pci@0000:00:00.0
          version: 00
          width: 32 bits
          clock: 66MHz
        *-pci:0
             description: PCI bridge
             product: RD790 PCI to PCI bridge (external gfx0 port A)
             vendor: ATI Technologies Inc
             physical id: 2
             bus info: pci@0000:00:02.0
             version: 00
             width: 32 bits
             clock: 33MHz
             capabilities: pci pm pciexpress msi ht normal_decode bus_master cap_list
             configuration: driver=pcieport
             resources: irq:40 ioport:a000(size=4096) memory:f8000000-fbbfffff ioport:d0000000(size=268435456)
           *-display
                description: VGA compatible controller
                product: G92 [GeForce GTS 250]
                vendor: nVidia Corporation
                physical id: 0
                bus info: pci@0000:01:00.0
                version: a2
                width: 64 bits
                clock: 33MHz
                capabilities: pm msi pciexpress vga_controller bus_master cap_list rom
                configuration: driver=nvidia latency=0
                resources: irq:18 memory:fa000000-faffffff memory:d0000000-dfffffff memory:f8000000-f9ffffff ioport:ac00(size=128) memory:fbbe0000-fbbfffff
        *-pci:1
             description: PCI bridge
             product: RD790 PCI to PCI bridge (PCI express gpp port C)
             vendor: ATI Technologies Inc
             physical id: 6
             bus info: pci@0000:00:06.0
             version: 00
             width: 32 bits
             clock: 33MHz
             capabilities: pci pm pciexpress msi ht normal_decode bus_master cap_list
             configuration: driver=pcieport
             resources: irq:41 ioport:b000(size=4096) memory:fbc00000-fbcfffff ioport:f6f00000(size=1048576)
           *-network
                description: Ethernet interface
                product: RTL8111/8168B PCI Express Gigabit Ethernet controller
                vendor: Realtek Semiconductor Co., Ltd.
                physical id: 0
                bus info: pci@0000:02:00.0
                logical name: eth0
                version: 03
                serial: e0:cb:4e:ba:c2:9a
                size: 10MB/s
                capacity: 1GB/s
                width: 64 bits
                clock: 33MHz
                capabilities: pm msi pciexpress msix vpd bus_master cap_list rom ethernet physical tp mii 10bt 10bt-fd 100bt 100bt-fd 1000bt 1000bt-fd autonegotiation
                configuration: autonegotiation=on broadcast=yes driver=r8169 driverversion=2.3LK-NAPI duplex=half latency=0 link=no multicast=yes port=MII speed=10MB/s
                resources: irq:45 ioport:b800(size=256) memory:f6fff000-f6ffffff memory:f6ff8000-f6ffbfff memory:fbcf0000-fbcfffff
        *-pci:2
             description: PCI bridge
             product: RD790 PCI to PCI bridge (PCI express gpp port D)
             vendor: ATI Technologies Inc
             physical id: 7
             bus info: pci@0000:00:07.0
             version: 00
             width: 32 bits
             clock: 33MHz
             capabilities: pci pm pciexpress msi ht normal_decode bus_master cap_list
             configuration: driver=pcieport
             resources: irq:42 ioport:c000(size=4096) memory:fbd00000-fbdfffff
           *-firewire
                description: FireWire (IEEE 1394)
                product: VT6315 Series Firewire Controller
                vendor: VIA Technologies, Inc.
                physical id: 0
                bus info: pci@0000:03:00.0
                version: 00
                width: 64 bits
                clock: 33MHz
                capabilities: pm msi pciexpress ohci bus_master cap_list
                configuration: driver=firewire_ohci latency=0
                resources: irq:19 memory:fbdff800-fbdfffff ioport:c800(size=256)
        *-pci:3
             description: PCI bridge
             product: RD790 PCI to PCI bridge (PCI express gpp port E)
             vendor: ATI Technologies Inc
             physical id: 9
             bus info: pci@0000:00:09.0
             version: 00
             width: 32 bits
             clock: 33MHz
             capabilities: pci pm pciexpress msi ht normal_decode bus_master cap_list
             configuration: driver=pcieport
             resources: irq:43 ioport:d000(size=4096) memory:fbe00000-fbefffff
           *-ide
                description: IDE interface
                product: 88SE6121 SATA II Controller
                vendor: Marvell Technology Group Ltd.
                physical id: 0
                bus info: pci@0000:04:00.0
                version: b2
                width: 32 bits
                clock: 33MHz
                capabilities: ide pm msi pciexpress bus_master cap_list
                configuration: driver=pata_marvell latency=0
                resources: irq:17 ioport:dc00(size=8) ioport:d880(size=4) ioport:d800(size=8) ioport:d480(size=4) ioport:d400(size=16) memory:fbeffc00-fbefffff
        *-storage
             description: SATA controller
             product: SB700/SB800 SATA Controller [IDE mode]
             vendor: ATI Technologies Inc
             physical id: 11
             bus info: pci@0000:00:11.0
             logical name: scsi0
             logical name: scsi2
             version: 00
             width: 32 bits
             clock: 66MHz
             capabilities: storage msi ahci_1.0 bus_master cap_list emulated
             configuration: driver=ahci latency=64
             resources: irq:44 ioport:9000(size=8) ioport:8000(size=4) ioport:7000(size=8) ioport:6000(size=4) ioport:5000(size=16) memory:f7fffc00-f7ffffff
           *-disk:0
                description: ATA Disk
                product: ST31000528AS
                vendor: Seagate
                physical id: 0
                bus info: scsi@0:0.0.0
                logical name: /dev/sda
                version: CC38
                serial: 9VP3WD9Z
                size: 931GiB (1TB)
                capabilities: partitioned partitioned:dos
                configuration: ansiversion=5 signature=000ad206
              *-volume:0 UNCLAIMED
                   description: Linux filesystem partition
                   vendor: Linux
                   physical id: 1
                   bus info: scsi@0:0.0.0,1
                   version: 1.0
                   serial: 81839235-21ea-4853-90a4-814779f49000
                   size: 972MiB
                   capacity: 972MiB
                   capabilities: primary ext2 initialized
                   configuration: filesystem=ext2 modified=2010-12-06 18:32:58 mounted=2010-11-01 07:05:10 state=unknown
              *-volume:1 UNCLAIMED
                   description: Linux swap volume
                   physical id: 2
                   bus info: scsi@0:0.0.0,2
                   version: 1
                   serial: 22b881d5-6f5c-484d-94e8-e231896fa91b
                   size: 486MiB
                   capacity: 486MiB
                   capabilities: primary nofs swap initialized
                   configuration: filesystem=swap pagesize=4096
              *-volume:2 UNCLAIMED
                   description: EXT3 volume
                   vendor: Linux
                   physical id: 3
                   bus info: scsi@0:0.0.0,3
                   version: 1.0
                   serial: ad5b0daf-11e8-4f8f-8598-4e89da9c0d84
                   size: 47GiB
                   capacity: 47GiB
                   capabilities: primary journaled extended_attributes large_files recover ext3 ext2 initialized
                   configuration: created=2010-02-16 20:42:29 filesystem=ext3 modified=2010-11-29 17:02:34 mounted=2010-12-06 18:32:50 state=clean
              *-volume:3 UNCLAIMED
                   description: Extended partition
                   physical id: 4
                   bus info: scsi@0:0.0.0,4
                   size: 882GiB
                   capacity: 882GiB
                   capabilities: primary extended partitioned partitioned:extended
                 *-logicalvolume UNCLAIMED
                      description: Linux filesystem partition
                      physical id: 5
                      capacity: 882GiB
           *-disk:1
                description: ATA Disk
                product: ST31000528AS
                vendor: Seagate
                physical id: 1
                bus info: scsi@2:0.0.0
                logical name: /dev/sdb
                version: CC38
                serial: 9VP3SCPF
                size: 931GiB (1TB)
                capabilities: partitioned partitioned:dos
                configuration: ansiversion=5 signature=000ad206
              *-volume:0 UNCLAIMED
                   description: Linux filesystem partition
                   vendor: Linux
                   physical id: 1
                   bus info: scsi@2:0.0.0,1
                   version: 1.0
                   serial: 81839235-21ea-4853-90a4-814779f49000
                   size: 972MiB
                   capacity: 972MiB
                   capabilities: primary ext2 initialized
                   configuration: filesystem=ext2 modified=2010-12-06 18:32:58 mounted=2010-11-01 07:05:10 state=unknown
              *-volume:1 UNCLAIMED
                   description: Linux swap volume
                   physical id: 2
                   bus info: scsi@2:0.0.0,2
                   version: 1
                   serial: 22b881d5-6f5c-484d-94e8-e231896fa91b
                   size: 486MiB
                   capacity: 486MiB
                   capabilities: primary nofs swap initialized
                   configuration: filesystem=swap pagesize=4096
              *-volume:2 UNCLAIMED
                   description: EXT3 volume
                   vendor: Linux
                   physical id: 3
                   bus info: scsi@2:0.0.0,3
                   version: 1.0
                   serial: ad5b0daf-11e8-4f8f-8598-4e89da9c0d84
                   size: 47GiB
                   capacity: 47GiB
                   capabilities: primary journaled extended_attributes large_files recover ext3 ext2 initialized
                   configuration: created=2010-02-16 20:42:29 filesystem=ext3 modified=2010-11-29 17:02:34 mounted=2010-12-06 18:32:50 state=clean
              *-volume:3 UNCLAIMED
                   description: Extended partition
                   physical id: 4
                   bus info: scsi@2:0.0.0,4
                   size: 882GiB
                   capacity: 882GiB
                   capabilities: primary extended partitioned partitioned:extended
                 *-logicalvolume UNCLAIMED
                      description: Linux filesystem partition
                      physical id: 5
                      capacity: 882GiB
        *-usb:0
             description: USB Controller
             product: SB700/SB800 USB OHCI0 Controller
             vendor: ATI Technologies Inc
             physical id: 12
             bus info: pci@0000:00:12.0
             version: 00
             width: 32 bits
             clock: 66MHz
             capabilities: ohci bus_master
             configuration: driver=ohci_hcd latency=64
             resources: irq:16 memory:f7ffd000-f7ffdfff
        *-usb:1
             description: USB Controller
             product: SB700 USB OHCI1 Controller
             vendor: ATI Technologies Inc
             physical id: 12.1
             bus info: pci@0000:00:12.1
             version: 00
             width: 32 bits
             clock: 66MHz
             capabilities: ohci bus_master
             configuration: driver=ohci_hcd latency=64
             resources: irq:16 memory:f7ffe000-f7ffefff
        *-usb:2
             description: USB Controller
             product: SB700/SB800 USB EHCI Controller
             vendor: ATI Technologies Inc
             physical id: 12.2
             bus info: pci@0000:00:12.2
             version: 00
             width: 32 bits
             clock: 66MHz
             capabilities: pm debug ehci bus_master cap_list
             configuration: driver=ehci_hcd latency=64
             resources: irq:17 memory:f7fff800-f7fff8ff
        *-usb:3
             description: USB Controller
             product: SB700/SB800 USB OHCI0 Controller
             vendor: ATI Technologies Inc
             physical id: 13
             bus info: pci@0000:00:13.0
             version: 00
             width: 32 bits
             clock: 66MHz
             capabilities: ohci bus_master
             configuration: driver=ohci_hcd latency=64
             resources: irq:18 memory:f7ffb000-f7ffbfff
        *-usb:4
             description: USB Controller
             product: SB700 USB OHCI1 Controller
             vendor: ATI Technologies Inc
             physical id: 13.1
             bus info: pci@0000:00:13.1
             version: 00
             width: 32 bits
             clock: 66MHz
             capabilities: ohci bus_master
             configuration: driver=ohci_hcd latency=64
             resources: irq:18 memory:f7ffc000-f7ffcfff
        *-usb:5
             description: USB Controller
             product: SB700/SB800 USB EHCI Controller
             vendor: ATI Technologies Inc
             physical id: 13.2
             bus info: pci@0000:00:13.2
             version: 00
             width: 32 bits
             clock: 66MHz
             capabilities: pm debug ehci bus_master cap_list
             configuration: driver=ehci_hcd latency=64
             resources: irq:19 memory:f7fff400-f7fff4ff
        *-serial UNCLAIMED
             description: SMBus
             product: SBx00 SMBus Controller
             vendor: ATI Technologies Inc
             physical id: 14
             bus info: pci@0000:00:14.0
             version: 3c
             width: 32 bits
             clock: 66MHz
             capabilities: ht cap_list
             configuration: latency=0
        *-ide
             description: IDE interface
             product: SB700/SB800 IDE Controller
             vendor: ATI Technologies Inc
             physical id: 14.1
             bus info: pci@0000:00:14.1
             logical name: scsi5
             version: 00
             width: 32 bits
             clock: 66MHz
             capabilities: ide msi bus_master cap_list emulated
             configuration: driver=pata_atiixp latency=64
             resources: irq:16 ioport:1f0(size=8) ioport:3f6 ioport:170(size=8) ioport:376 ioport:ff00(size=16)
           *-cdrom:0
                description: DVD reader
                product: DVDROM DH16NS30
                vendor: HL-DT-ST
                physical id: 0.0.0
                bus info: scsi@5:0.0.0
                logical name: /dev/cdrom1
                logical name: /dev/dvd1
                logical name: /dev/scd0
                logical name: /dev/sr0
                version: 1.00
                capabilities: removable audio dvd
                configuration: ansiversion=5 status=nodisc
           *-cdrom:1
                description: DVD-RAM writer
                product: DVDRAM GH22NS50
                vendor: HL-DT-ST
                physical id: 0.1.0
                bus info: scsi@5:0.1.0
                logical name: /dev/cdrom
                logical name: /dev/cdrw
                logical name: /dev/dvd
                logical name: /dev/dvdrw
                logical name: /dev/scd1
                logical name: /dev/sr1
                version: TN02
                capabilities: removable audio cd-r cd-rw dvd dvd-r dvd-ram
                configuration: ansiversion=5 status=nodisc
        *-multimedia
             description: Audio device
             product: SBx00 Azalia (Intel HDA)
             vendor: ATI Technologies Inc
             physical id: 14.2
             bus info: pci@0000:00:14.2
             version: 00
             width: 64 bits
             clock: 33MHz
             capabilities: pm bus_master cap_list
             configuration: driver=HDA Intel latency=64
             resources: irq:16 memory:f7ff4000-f7ff7fff
        *-isa
             description: ISA bridge
             product: SB700/SB800 LPC host controller
             vendor: ATI Technologies Inc
             physical id: 14.3
             bus info: pci@0000:00:14.3
             version: 00
             width: 32 bits
             clock: 66MHz
             capabilities: isa bus_master
             configuration: latency=0
        *-pci:4
             description: PCI bridge
             product: SBx00 PCI to PCI Bridge
             vendor: ATI Technologies Inc
             physical id: 14.4
             bus info: pci@0000:00:14.4
             version: 00
             width: 32 bits
             clock: 66MHz
             capabilities: pci subtractive_decode bus_master
             resources: ioport:e000(size=4096) memory:fbf00000-fbffffff
           *-network
                description: Ethernet interface
                product: 82541PI Gigabit Ethernet Controller
                vendor: Intel Corporation
                physical id: 5
                bus info: pci@0000:05:05.0
                logical name: eth1
                version: 05
                serial: 00:1b:21:56:f3:60
                size: 100MB/s
                capacity: 1GB/s
                width: 32 bits
                clock: 66MHz
                capabilities: pm pcix bus_master cap_list rom ethernet physical tp 10bt 10bt-fd 100bt 100bt-fd 1000bt-fd autonegotiation
                configuration: autonegotiation=on broadcast=yes driver=e1000 driverversion=7.3.21-k6-NAPI duplex=full firmware=N/A ip=192.168.1.2 latency=64 link=yes mingnt=255 multicast=yes port=twisted pair speed=100MB/s
                resources: irq:20 memory:fbfe0000-fbffffff memory:fbfc0000-fbfdffff ioport:ec00(size=64) memory:fbfa0000-fbfbffff
        *-usb:6
             description: USB Controller
             product: SB700/SB800 USB OHCI2 Controller
             vendor: ATI Technologies Inc
             physical id: 14.5
             bus info: pci@0000:00:14.5
             version: 00
             width: 32 bits
             clock: 66MHz
             capabilities: ohci bus_master
             configuration: driver=ohci_hcd latency=64
             resources: irq:18 memory:f7ffa000-f7ffafff
     *-pci:1
          description: Host bridge
          product: Family 10h Processor HyperTransport Configuration
          vendor: Advanced Micro Devices [AMD]
          physical id: 101
          bus info: pci@0000:00:18.0
          version: 00
          width: 32 bits
          clock: 33MHz
     *-pci:2
          description: Host bridge
          product: Family 10h Processor Address Map
          vendor: Advanced Micro Devices [AMD]
          physical id: 102
          bus info: pci@0000:00:18.1
          version: 00
          width: 32 bits
          clock: 33MHz
     *-pci:3
          description: Host bridge
          product: Family 10h Processor DRAM Controller
          vendor: Advanced Micro Devices [AMD]
          physical id: 103
          bus info: pci@0000:00:18.2
          version: 00
          width: 32 bits
          clock: 33MHz
     *-pci:4
          description: Host bridge
          product: Family 10h Processor Miscellaneous Control
          vendor: Advanced Micro Devices [AMD]
          physical id: 104
          bus info: pci@0000:00:18.3
          version: 00
          width: 32 bits
          clock: 33MHz
          configuration: driver=k10temp
          resources: irq:0
     *-pci:5
          description: Host bridge
          product: Family 10h Processor Link Control
          vendor: Advanced Micro Devices [AMD]
          physical id: 105
          bus info: pci@0000:00:18.4
          version: 00
          width: 32 bits
          clock: 33MHz
     *-scsi
          physical id: 1
          bus info: usb@2:3
          logical name: scsi8
          capabilities: emulated scsi-host
          configuration: driver=usb-storage
        *-disk:0
             description: SCSI Disk
             physical id: 0.0.0
             bus info: scsi@8:0.0.0
             logical name: /dev/sdc
        *-disk:1
             description: SCSI Disk
             physical id: 0.0.1
             bus info: scsi@8:0.0.1
             logical name: /dev/sdd
        *-disk:2
             description: SCSI Disk
             physical id: 0.0.2
             bus info: scsi@8:0.0.2
             logical name: /dev/sde
        *-disk:3
             description: SCSI Disk
             physical id: 0.0.3
             bus info: scsi@8:0.0.3
             logical name: /dev/sdf
  *-network DISABLED
       description: Ethernet interface
       physical id: 1
       logical name: vboxnet0
       serial: 0a:00:27:00:00:00
       capabilities: ethernet physical
       configuration: broadcast=yes multicast=yes