When does this happen? After you've been playing games? Or just completely randomly?
It would really help us if you could tell us more info. You don't just tell the car mechanic, "The engine stalled." Well, you DO if that's really all that happened, but something more like, "The car started shuddering, and making a screeching sound and THEN it stalled. And it was really cold when it happened."
All I can say is make sure it's not overheating. The build-up of dust over the years can lead to parts overheating and causing that problem. If it STILL does it, you might want to reseat the RAM and processor if you know how to do that. Running MemTest might also be advised to see if it's the RAM. Of course, it could also be software related. If you installed something recently, that might be it.