Shaders,framebuffer help!

I'm working on a heat haze effect i got some problems,  I have to grab the frame buffer each frame which is killing my fps.  The shader alone drops the scene from ~300fps to about 20fps.  Gefore4 Ti 4200 just isn't cutting it.  any ideas?  or if somebody (with current cvs) is interested, private message me and i'll send you the source.