Clean out all of the built up gunk in the interior of the computer and make sure the interior fans are clear of build up so that it is getting the proper air flow.
To take it a step farther, you can take off the side panel and put a giant fan right up to it, which is what I do. If it's still too hot you can try putting ice in front of the fan like an old timey air conditioner.
Just reach in and touch the graphics card when you are having this problem and if it's super hot to the touch, this may be the issue.