Dropping fps, that remains low when AI count goes down.

I have a problem that I hope you guys can help me shed some light on.. in my mission I have up to 90 AI and 50 AI in dynamic simulation and this drops my fps to between 30-40. After an hour or two I am down to 20+ at some times. I figure this is when a lot of the AI is involved in fighting. But then it never gets up, it stays steady around 20, and even in my endgame when the AI is down to -60-70 by fps doesnt go back up?So my thought is that there are some garbage, some threads that keep spinning or something similiar, but I cannot figure out what! Anyone knows anything about this? Is there something I can do to "clean" up stuff?