Maybe its the Dynamic Lighting? Go to 'more video' in the setup menu and be sure all the fancy lighting is disabled, and maybe turn down the detail a bit. Also, make sure the shadows are on simple or none. Vulmetric kills systems! Check in your Video options for the resolution you are running it on. 800x600 and below run pretty good, even though it will look a tad choppy. Even turning your detail settings to medium makes a difference.
Also, patch your game.
And if nothing that I said helps, go to Yoda's Help Desk (
http://yodashelpdesk.lucasarts.com/default.asp?sid=215238050927067186019096) at lucasarts.com and try it. They usually help. And if THAT don't work, just call them. They're pretty nice and try to help yuo as best as they can (and they usually succeed.)
Hope I helped. :D