Did you ever figure this out @Bherm? I'm having a similar problem. Frame took a huge dump. Restart temporarily fixes it. Good frame rate. Do a couple races and it drops down again from mid 130's to 65 fps. I've tried turning on and off Shader's Patch, downgrading, reinstalling Shaders, reinstalling Sol. Ugn. Interestingly when running a race in this state with Shaders off, it is still in the dumps. Please don't tell me I have to do the old integrity check...
Sort of, I was going nuts trying to figure out the issue. Never happened once before upgrading from 0.1.60 to 0.1.62 and even going back to 0.1.60 afterward I was still experiencing the same thing. I then started to think it might be a RAM cache issue or some kind of memory leak in the programming, tried some memory optimization tools, still happened. Finally, I noticed whenever it acts up with the giant FPS drop, the first hint is that the loading time will increase and I noticed when loading the track and cars, it would come to crawl and each car would take much longer to load. In my desperate attempt, I started to think maybe this could be a HDD related issue. Since I have a M.2 SSD, I ran the optimization tool that came with my SSD and so far knock on wood, even on 0.1.62 it hasn't happened again. Tracks and cars that I normally get 80 to 75fps is holding steady, before with this giant bug, I would dip into the 40s FPS range...I am convinced it has something to do with the latest CSP 0.1.62 that broke something even if you go back to the previous version. Hopefully this SSD optimization fix will be good enough for now.
Also, deleted Camtool and all other apps before this fix and didn't make a difference, also no problem with any other racing games out there (F1 2020, AM2, ACC) and double check to make sure it wasn't some malware running in the background sucking up CPU cycles