Monster Hunter Tri:Slowdown

[Deep Sea Diver]

I've been using build 5423M and Ive been getting slowdown (20-25fps) when Ive got alot of the things on screen (two wyverns + jaggi/baggi). Is anyone else able to maintain 30fps when this happens? If so, could you post the build you are using and your config.

My PC specs: C2Q 9550
gtx 260
4g RAM
Vista x64