To expand on what dug777 said, it can also happen when a system doesnt have enough memory. Its not always video. BF2 is a good example of this. With just one gig, and settings up on high, the system will "hitch" for the first few minutes, then it will smoothen out. It will still hitch from time to time, but not as bad. F.E.A.R. is the same way, but not as bad. Generally it does it when you enter a room, or go from inside, to outside. When you dont have enough memory, it has to swap.