If you save the game does the performance go back to normal for awhile?
I had that problem once (there are a couple similar topics), but only with a bunch of background programs and not having rebooted my computer for days. After rebooting I couldn't reproduce the drop with a 'normal' amount of background programs running (several Explorer, text editor and web browsers open).
There is a separate performance issue some people have with laptops using dual graphics.
