Ok, I've isolated the problem. 99.9% of the times it happens under XP. Has only occured once while in OSX.
I've also determined that it only happens when I'm playing a game. Boot up FFXI, and it either shuts down halfway through loading up or a few seconds into play. The exact same thing happens for Morrowind and Ragnarok.
These are the only games I play.
It's not because the games are too intensive or anything. Ragnarok is 2d for goodness sakes.
Why the hell does this keep happening. I want to take it to the genius bar. But what do I say? "Hey, my mac shuts down when I run windows".
ANY suggestions/theories?