Yeah, I think it's a problem across a majority of stalker mods. As you say, a restart sorts it out every time, but it would be nice to find a work around for it. :/
I've been playing a lot of this mod over the last few days, and some of the fps drops are crazy. I've seen it go as low as 8 fps at one point.
I've now tried turning off sun rays / AO & or SSAO. The fps hasn't dropped at all, and to tell the truth I can hardly tell the difference (if at all). It does also mean that I can now crank up the AA, though pushing it to the max doesn't make it look 'that' much better, and the frame rate hit is huge. I find sticking with morphological AA to be a good enough trade off.