Oddly enough, I have the same trouble with my Radeon 9800.

Whenever I do something, such as play a video game, or sometimes even when I don't do anything, the computer will freeze, or perform a VPU recovery.

I've always attributed this to over heating of the card, since I have the most up-to-date drivers, and it seems to occur less when I have an external fan running on it.

But maybe I'm wrong, and there's a reason for it?