Yeah I had the same problem. Try also to unclick the EAX box under SOUND.
I found that this turn off most of the sound and made the game slow. I use to have a Radeon x700 256mb PCi-E card and no matter what I did to lower the settings it was still jumpy. In the end I bought another VGA card (Nvidia 7600GT 256mb)AU$285. Every game I run is so much more better. Also try updatin' your VGA Drivers. I hope this helps.