I have got two ubuntu 12.04 servers(xeon e3). Sometimes(several days) each server spontaneously reboots. HDDs and other hardware are ok.

Which logfile can help find a reason of the problem?

UPDATED.

hardware: xeon e3 processor, intel server motherboard, 32gb ddr3 ecc, mdadm mirror hdd raid for system, mdadm ssd raid for database(postgres).

Both servers have similar (not identical) components. Smart is OK. It seems that the problem is in the software. Python process and database are running on this servers.

Syslog (time of reboot):

Aug 23 13:42:23 xeon hddtemp[1411]: /dev/sdc: WDC WD15NPVT-00Z2TT0: 34 C
Aug 23 13:42:23 xeon hddtemp[1411]: /dev/sdd: WDC WD15NPVT-00Z2TT0: 34 C
Aug 23 13:43:24 xeon hddtemp[1411]: /dev/sdc: WDC WD15NPVT-00Z2TT0: 34 C
Aug 23 13:43:24 xeon hddtemp[1411]: /dev/sdd: WDC WD15NPVT-00Z2TT0: 34 C
Aug 23 13:44:14 xeon sensord: Chip: acpitz-virtual-0
Aug 23 13:44:14 xeon sensord: Adapter: Virtual device
Aug 23 13:44:14 xeon sensord:   temp1: 27.8 C
Aug 23 13:44:14 xeon sensord:   temp2: 29.8 C
Aug 23 13:44:14 xeon sensord: Chip: coretemp-isa-0000
Aug 23 13:44:14 xeon sensord: Adapter: ISA adapter
Aug 23 13:44:14 xeon sensord:   Physical id 0: 37.0 C
Aug 23 13:44:14 xeon sensord:   Core 0: 37.0 C
Aug 23 13:44:14 xeon sensord:   Core 1: 37.0 C
Aug 23 13:44:14 xeon sensord:   Core 2: 37.0 C
Aug 23 13:44:14 xeon sensord:   Core 3: 37.0 C
Aug 23 13:44:24 xeon hddtemp[1411]: /dev/sdc: WDC WD15NPVT-00Z2TT0: 34 C
Aug 23 13:44:24 xeon hddtemp[1411]: /dev/sdd: WDC WD15NPVT-00Z2TT0: 34 C
Aug 23 13:47:01 xeon kernel: imklog 5.8.6, log source = /proc/kmsg started.
Aug 23 13:47:01 xeon rsyslogd: [origin software="rsyslogd" swVersion="5.8.6" x-pid="582" x-info="http://www.rsyslog.com"] start
Aug 23 13:47:01 xeon rsyslogd: rsyslogd's groupid changed to 103
Aug 23 13:47:01 xeon rsyslogd: rsyslogd's userid changed to 101
Aug 23 13:47:00 xeon rsyslogd-2039: Could not open output pipe '/dev/xconsole' [try http://www.rsyslog.com/e/2039 ]
Aug 23 13:47:01 xeon kernel: [    0.000000] Initializing cgroup subsys cpuset
Aug 23 13:47:01 xeon kernel: [    0.000000] Initializing cgroup subsys cpu
Aug 23 13:47:01 xeon kernel: [    0.000000] Initializing cgroup subsys cpuacct
Aug 23 13:47:01 xeon kernel: [    0.000000] Linux version 3.11.0-26-generic (buildd@komainu) (gcc version 4.6.3 (Ubuntu/Linaro 4.6.3-1ubuntu5) ) #45~precise1-Ubuntu SMP Tue Jul 15 04:02:35 UTC 2014 (Ubuntu 3.11.0-26.45~precise1-generic 3.11.10.12)
Aug 23 13:47:01 xeon kernel: [    0.000000] Command line: BOOT_IMAGE=/boot/vmlinuz-3.11.0-26-generic root=UUID=0daa7f53-6c74-47d2-873e-ebd339cd39b0 ro splash quiet vt.handoff=7
Aug 23 13:47:01 xeon kernel: [    0.000000] KERNEL supported cpus:
Aug 23 13:47:01 xeon kernel: [    0.000000]   Intel GenuineIntel
Aug 23 13:47:01 xeon kernel: [    0.000000]   AMD AuthenticAMD
Aug 23 13:47:01 xeon kernel: [    0.000000]   Centaur CentaurHauls
Aug 23 13:47:01 xeon kernel: [    0.000000] e820: BIOS-provided physical RAM map:
Aug 23 13:47:01 xeon kernel: [    0.000000] BIOS-e820: [mem 0x0000000000000000-0x000000000009bbff] usable
Aug 23 13:47:01 xeon kernel: [    0.000000] BIOS-e820: [mem 0x000000000009bc00-0x000000000009ffff] reserved

Dmseg - nothing strange.