My computer keeps rebooting at random times. It will go several days and reboot or it will reboot 3 times in 1 day, you never know! There's never an error when it comes back up and the only thing of interest I've found in the system logs has been a critical kernel-power error: "The system has rebooted without cleanly shutting down first. This error could be caused if the system stopped responding, crashed, or lost power unexpectedly."
As a note, I've already run the RAM through its paces with MemTest.