Interestingly my GTA is running fine and i get a solid 60fps, However when im bolting through the map at high speed i seem to hit "loading areas" where it goign from 60fps then rapidly down to 30ish fps for a second or two then back up to 60fps again. I was wondering if this had anything to do with it not having enough Ram as its the only thing when running the game that is near capacity.
I had something similar and found moving the game to my SSD solved it for me. I get smooth frames all of the time now.