Start on absolutely lowest detail-levels, and work your way up......might be the graphics card isn't coping with rendering something in the game, could be anything really.

Also, if you're on broadband, download the following file which will benchmark all necessary parts of your computer (should show you what's causing the slow-down - don't bother if you're on dial-up, though, it's a 177Mb download):

http://www.download.com/3DMark03/300...ml?tag=lst-0-5