I have the same problem with a Radeon X1950, Driver Version 8.591.0.0 / Catalyst which I just downloaded [XP Prof.]. I noticed that this effect will occur after some time. When I quit the game to the desktop with Alt-Tab and return to the game, everything is fine for a while. It doesn't seem to be a heat problem though (its about 72C). And still this effect is not very predictable because I played 1h straight without any effect.