I'm playing lots of other games on the same PC and they are fine:
Dangerous Waters, Counter-Strike 1.6, IL-2 Shturmovik: 1946, The elder scrolls: Oblivion, Operation Flashpoint: Resistance.
I did have this very problem on some fitness program that my mom installed recently, but I did fix that somehow in the program's settings.
Cleaning the heatsink helps? GPU core temp is in optimum levels. And this solution also sounds like a grass straw for the drowning. I don't want to be dissapointed if it doesnt help.
Update:
Cleaned the heatsink (it was clean... but whatever), installed an old cooler on top of it, but it doesn't help. at all. All that has increased is the noise level, no improvement in visual quality of the game.