Hmm, i got SP1 and it runs ok...
Have you tried
www.gtaforums.com?
General feeling is that there are all sorts of driver issues and even rolling back can help. I have the 9800 Pro too, and am running the latest Omega drivers currently based on the cat5.4 drivers. (Current ATI drivers are 5.6 i think..) So you could try rolling back to the omega or ati 5.4 drivers...
One person with an nVidia card had to rollback directx to 9.0b, but I dont recommend that!
Hope this helps :thumbup: (good username btw!)