Why does COD keep freezing on me?

I have my system moded and my COD: Black Ops is a burned game. It always freezes on me when I am playing solo on zombies. It only freezes on me in the map Kino Der Toten. The room with the projector after you use the teleporter. It won't freeze on me at any specific round, but it freezes on me up there. Why does it do this, and is there a way to fix the problem?