I have the same kinda problem. I installed LFS on my computer today, and when I started playing I constantly had 100 fps. I set the graphics higher, and still got around 60 fps.
But now, for some reason framerate keeps jumping around between 5 and 100 fps, no matter how low I set the graphics. Screen might actually freeze for a second or two, and then I suddenly get 60 fps again. Problem is not overheating, and not antivirus either. I don't have any other applications running at the same time. And I certainly have a fast enough computer for LFS.
Any ideas?
EDIT: this problem only applies to actually driving, no framerate drop at all in replays.
EDIT2: More info about my case. Restarting my computer helps, everything is ok for a while. But after playing for a couple of hours, framerate starts jumping again. I think I'll do some memory monitoring, maybe there's a memory leak or something like that?