The power supply is a suspect.
But before you try anything else, you need to prove that the problem is not "overheating".
Remove the side panel from the computer and set a house fan where it will blow into the computer. Run the computer with the side removed (external fan blowing) and see it the problem changes or reoccurs.
I built a 3 Ghz P4 system @ 800Fsb, 400W power supply, with a Gigabyte GA-8S648FX motherboard, 512MB PC3200 RAM, and a Radeon 9600 graphics card.
It's been running fine, but NOW, without warning, it blacks out and shuts off in the middle of working. I try to click the power button to turn it back on.. and NOTHING! dead.
if I remove the battery on the Mboard, and reinstall it, the BIOS is reset and the system powers up again. until a few hours/days later, out of the blue.. it will do the same thing and I need to uninstall and reinstall the battery to reset the BIOS. ???????
there seems to be no performance issues before the comp shorts out, so I do not think it's my MB/RAM/or CPU. Power is still supplied to my optical mouse even when it blacks out, so I am skeptical if it's my power supply, but I have heard of crappy PSs being shipped with custom cases.
any one experienced this? any thoughts?