FPS Drops

I was hoping someone could help me out on this. At several of the SBP, Revamped, & FSE tracks I run, I experience a drop from around 145FPS to 4FPS, just off the corners. I experience it, worst at Rockingham Revamped 2012 & Auto Club 2014. It seems to happen intermittently. For example, I might run 15 laps with no drop and then the next 5 will be junk. I have turned on/off objects, changed mods, +/- Cars Drawn & % drawn ahead. Nothing seems to help it. Any ideas?

FYI. I'm running a NVIDIA...

FPS Drops