Are you running an NT based OS (Win2K, XP) ?
They suck at games especially windows2000 (worst POS ever released). There's a lot of disk access when loading. When did you last defrag that 80 gig?
Have you turned off some video options. The scrambled video before the 1.2 patch was caused by anti-aliasing or video textures (I can't remember which). Turn off everything that you can and see if it helps.
Video drivers are funny. Often an older driver works better than a newer one. Nvida detonator drivers versions after 41.09 seem screwed up. Try using 30.82. It seems to work perfectly with MMIX. I think that 30.82 is the last one released that has been approved by Microsoft as fully DirectX compatible.