Computer powers off randomly

Running whitebox system, Windows XP SP2. AMD Athlon 64, 2GB RAM. Problem: computer will suddenly turn off and will not turn back on unless I physically unplug the computer and plug it back in or on/off the button on the power supply. Power button will not turn it on. It happens at random (from an hour to 2 days). 90% of the time, it's when I'm burning a DVD with Nero Vision, but will happen randomly throughout the process (sometimes 5 minutes in, usually right near the end (30-45 mins in)). I can't catch exactly where in the process it's turning off.

I took all the components out of the box and ran in on a table thinking maybe a short, and it worked fine. I tested the outlet: worked fine. Tested on diff outlets: fine. Temps are good. Dusted it. Mem test fine. I've shaken/jostled the computer while the program was running to test for a short, but nothing.

Any ideas? Thanks!

 

Power supply old or under-rated for your system?

 

have you added any new hardware?? what is the exact rating of your psu? what athlon chip do you have??
you cant check the system temps from the bois. the system needs to be under load. like burning a dvd or streaming video. the system is in an idle state while viewing the bios.