I'm sorry to say I don't have any suggestions either than what Gman posted about the latest drivers, but I wanted to include my own experience with the game.
I don't have what anyone would call a high-end system, but I was able to play the game on max settings, with my fps never dropping below 50fps even in the built up areas. i5 750, XFX 5770, 4GB ram, etc... Not exactly a high-end system.
So with your setup (especially the 6990 you shouldn't be having that kind of trouble). I hope you find your answer.