FPS lock

I was wondering if there's a way to lock fps in crossfire with some gpu settings, just like in cs.
I think almost every player suffer with fps drop in some bad situations, just like when you're rushing.. the game perfomance clearly decreases, though it's not an issue.. the gpu works 100% all the time, as there are less things for it to render, your fps will increase and when you really need it, there is a fps drop.
In my case, my fps is about 150, though when i'm being shot it drops to 80.. it's not a bad fps at all, but the fast drop makes the game lag.
Thanks in advance.