I've had this problem for a while. It resurfaced recently after installing Fedora Core 6.
Basically the machine seems to randomly power-off. No warning, nothing. Just switches off. I have ran memtest86, no problems there. The power supply is generating enough, the voltages all seem okay. It seems to happen at random, there is no associated process, or action which causes it. Motherboard temps seem okay.
THe system is:
AMD Athlon64 3700
2 x 512mb Corsair Value RAM
ECS KN1 Extreme motherboard (with nforce4 chipset)
480w Hiper psu
Times it has happened:
It used to happen a lot while running Gentoo linux, so at first I thought that perhaps I had "overoptimised" the OS. However it does periodically happen while in Windows XP. It was happening every time I booted Fedora COre 6 from Grub. It didn't even get to load the kernel, just switched off. It most recently happened while updating packages in Fedora.
It isn't made worse when I am running intensive programs such as games. I just can't figure this one out.
Does anyone have any ideas?