I have also this problem. It's quit annoying. Can't play the U-drive it missions anymore because of it(it looks like very bad lag). And when I push the pause button, the cars are still moving, in what can be described as fast forward. Everything is also installed with the latest drivers and the latest patch of the game. I don't have any problems with other games, just with SC4. (yes, even the original)
I can tell that its no problem with an nVidia card. I play the game now for 3 years with an nVidia card, but just recently I put a new motherboard in my PC, with a dual core processor. The X2 4400 of AMD. Note that I'm not running as a server.
Does anyone know how to change the processor so that it runs normal again?