I had similar problems on my machine when I was playing...well anything really, but the more graphically demanding the game the worse it was.
Hopefully you do not have the same issue I had. In my case the problem came from a dying video card, which turned out to be due to a dying power supply (figured that out after I burned up two more VC's)
I ended up rebuilding my machine and upgrading to Win 7.
Have you had issues with any other graphic intensive games (Oblivion or WoW are good tests). Does it start out fine and gradually get worse? Does it happen in a single area or in all areas in the game.
Let us know.
