Not sure what happened, but it doesn't seem to have to do with the recent updates. It started (again?) a few days ago. It could possibly have something to do with the new-ish 326.19 Nvidia beta drivers, but idk. I've been using 720p terrain and it's not started by a vehicle, so it doesn't seem to be that issue I had before. However, I do still wonder if I've reached whatever that possible performance issue with roads is that I brought up quite a while ago, back before I restarted, but... this one seems like it might be related to the 3D grass perhaps (and the beta drivers?).
Only happens when I'm near the ground. My framerate drops from my normal 55ish-60 to 45-42 with frequent spiked drops to the mid to lower 30s, and it gets very choppy and stuttery, only when I'm close to the ground.