Sometimes, my Linux PC unexpectedly closes my session (log out) while I am working, and puts me back the login window. If I have terminal 1 logged with root and 2 logged with another user (without X), these ones are not logged out. For example, for the last three hours, it has happened five times. Other times, it leads to a reboot. So I have investigated: the UPS, but the behaviour is unchanged without it). the temperatures, but everything is fine (cpu, disks, video card). I have started to investigate also: the memory (partially tested - I have to run a full testyt next night). EDIT: totally tested and passed. My configuration is: Linux 2.6.37-tuxonice x86_64 Intel(R) Core(TM) i7 CPU 860 @ 2.80GHz. Any idea please?