I used to have this problem with my 6600GT while playing HL2. However, in my case it seemed to be completely random. I don't believe my card was overheating. My best guess was that either some pipes in the GPU or some of the memory modules were buggy and couldn't take the high clock speeds. Eventually my card blew up. I don't know if the blowing up had anything to do with this problem, though.
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Hmm, thanks I guess.