This can be caused by many things including out-of-date drivers, your video card is overheating (how do other games perform over time?), power requirements not being met for the video card when gaming, driver conflicts, and there's always the question of, can you give us a hint? Are you using multiple monitors? Are you in SLI mode(multiple video cards)? What is the specifications of your machine, and what is your video card temperature running at when you are playing? Have you tried a different resolution?
Need more hints.