that's strange, you're computer appears to be pretty top notch. Defragging might help, but maybe you've simply got too many other things running in the background, but that's just a flat out guess. The best way to figure out these kind of things is troubleshoot anything you can think of. THis might be a stupid suggestion, but have you tried simply rebooting your system? Hope that helps.
is it on your hardrive or on cd? i got mine from a friend on cd and it played like shit when i tried streaming it off the disk. if thats what your doing try copying it to the hardrive.