We're going to need a lot more information.
What programs are having the problem?
Does it happen in ALL graphics-intesive programs or just certain ones?
When did it start happening and what has changed since then?
What are the specs on your PC, specifically, CPU, RAM, video card.
What OS are you running? What service pack for the OS?
What version of the Chipset and Video Card drivers do you have installed? (not just "most recent" but actual version numbers)