probably a driver conflict.....
or your processor's overheating.
if it's a p4/celeron from the p4 era, they actually throttle themselves when they're approaching their thermal limit, and cause very choppy performance. and since it doiesn't have a voltage-adjustment throttling ability like the AMD cool n' quiet, or a laptop processor, instead of just slowing the processor down when it gets too hot, it throttles it by rapidly pausing, and unpausing the processor, which makes your computer extremely jerky