MAMEWorld Forums - Hardware - Stumped by rebooting Shuttle XPC

I have a Shuttle XPC (SA76G2) with an Athlon II X2 250 that I built (for my mom) in 2009. It worked great for several years, but it's suddenly started rebooting randomly on her.

It seems to happen when the CPU and/or GPU are stressed. It'll idle happily for hours, but even just loading up a Flash game or video will cause it to abend in 5-10 minutes.

It's *not* a bluescreen. I've disabled auto-restart, and it still just reboots without warning. Event Viewer shows nothing in the logs, other than a note that the previous shutdown was abnormal. When this happens, startup always fails just after POST with a keyboard error. Simply resetting the machine again will boot normally every time.

The first thing I did was replace a wireless keyboard with a basic, wired keyboard. When that didn't change anything, I also tried a known good wired keyboard. Still no change, so the keyboard error doesn't seem to be legit. The keyboards are all USB, which rules out a bad port or anything like that.

Windows 7 and drivers are all up to date as far as I can see. She's got up-to-date virus protection, and the machine is generally very clean, so a virus seems unlikely. I flashed the BIOS to one several revisions newer - no change. Running Memtest86 now, but it's already made it through one pass without errors, so I suspect it's not going to uncover anything.

I'm kind of running out of tests to run, and I hate to start replacing parts at semi-random. Where it seems to be a true hard reset, I'm guessing maybe the PSU or something power-related in the motherboard? Anyone seen anything similar? I can't say I've ever seen a true hard reset in a modern PC before, it's almost always an auto-restart from a bluescreen.