I feel pretty confident I know what it is.
I was ignorant a while back and was running my P4,R8500, and 4 IDE devices off of a 200watt PSU.
Checking the voltages, my voltages to my video card would drop...then CRASH.
this MIGHT be what your problem is, a little different then what I had, but still could be it...
my crashes werent always hard rebooting though, sometimes my music would still be going, but there would be lines on my screen..and I couldn't do anything.