ZM Crater HARD Special Round Fps Crash

Title pretty much sum's up my problem. Evertime I get to the special round on hard mode ZM Crater. my Fps goes from 50sih to a steady 15 an lower. The choppyness makes it pretty darn hard to shoot anything much less a warmonger mob. This is the only time,that my fps dies that badly.

Radeon AMD HD 4200 :Windows 7 Professional:AMD Phenom Proccesor Quad Core
all of the lowest settings on Crossfire.

any suggestion's or solutions to fix this would be greatly appreciated xD