Over the last few weeks, I have noticed that my Linux based file server will randomly reboot. The first time I noticed it I did not give it much thought, but now it happens about once a day. The symptoms are that it will be back at the login screen and there will be nothing the /var/log/messages to indicate that Linux saw a problem. I have so far removed the UPS from the problem. Its power is not glitching causing the problem. I am pretty sure the motherboard is not overtemp. I have a utility running that will email me if the temps go outside reasonable ranges. Over the summer, I would get the occasional warning on a hot day when the AC was off.
Tonight I will run a memory test (memtest286+) and if that passes, the next item on my list is the power supply (PSU). I will rig up my scope to trigger on the Power_Good signal wire from the PSU back to the motherboard. Perhaps the PSU is failing. If this does not fix the problem, I will disconnect the FurnaceMonitor USB to RS-485 device that was giving me grief before. Perhaps this is making the machine unstable?
After that, its a test of the system drive, swapping of the PSU, and then I guess a motherboard replacement. That I am not looking forward to.