You are probably running a 16x.xx driver. I would upgrade to a 17x.xx driver to eliminate this problem. I used to have it all the time (it was worst in UT3 and did happen in Crysis too) and the 17x.xx drivers got rid of the problem and performance increased. You can downgrade to 15x.xx or upgrade to 17x.xx if you want. It's all up to you.
You can find a wealth of performance charts, driver uploads, and tweaks over at
http://www.laptopvideo2go.com. They offer both laptop and desktop drivers. Alternatively, consider picking up some Omega drivers. However, I don't think these have made it to the 17x.xx series yet, but I may be wrong.