I currently have the motherboard of a Dell XPS 8930 in a Thermaltake CTE T500 case with a Corsair CX750M power supply.

The machine complains about a defective power switch cable on boot, because the Thermaltake's power button does not have the special Dell magic - but it had been running fine like that for months.

Recently, however, the machine has been rebooting once or twice a day, most of the time when I am not around - either in the middle of the night, or while I am at work.

I was in front of it once when it happened: The screen turned black for a moment, then the machine booted up. It was as though someone had pressed the reset button. The reset button, by the way is not connected, but the cable is tied away safely, and can definitely not touch any metal parts.

This started right after my UPS died; I had the PC plugged into the wall for a while, and that was when it happened first. It is now plugged into a UPS again, but that did not make a difference.

This far, I've verified that all connectors are firmly seated (including RAM and the video card) and reinstalled the OS. I've visually inspected the motherboard - no bulging capacitors or similarly scary stuff as far as I can tell.

I guess the next step would be to replace the power supply. It is quite new, but I suppose it could have gotten damaged, or just be a lemon.

Any cool ideas for other things I could try before I do this?