Due to the fact that it happens after a while in a game, I'd guess overheating on either the north/south bridge or the GCard itself.

If you can, point a fan at them and play a game, see if if fixes the problem, if you do it one by one you would hopefully be able to find out which one is causing the issue (Or use GPU-Z to log Graphics temperatures and concentrate the fans on the NB/SB, unless they have thermal sensors too (Which is quite rare))