Why is brinks FPS really bad on my 6970?

I recently got brink, and when i play offline i get a bad FPS jumpy from 45-60 and is really irratating, then i tried online and i get a much better FPS around 59-61? i dont know why? is this an in game problem or my system? my specs are AMD Phenom quad core @3.50ghz AMD radeon HD 6970, 1tb harddrive 7200rpm and 8gb RAM. Thank you