I dontget framerate drop, i just get a low fps of 239. Its odd becuase my set up is:

AMD Phenom II X4 965 Black edition 3.4GHz
Crossfirex 2x 1gb ATI radeon 5870

I should gets loads more since this is a new comp.

Hmm thats odd, my system isn't as good as that and my fps are higher, try updating your drivers and look for background programs that are eating up RAM.