I have GOTR installed (no optional mods) and am running the game on windows 8. However, after about 20-30 minutes of playing the fps drops to an unplayable level. strangely enough it only seems to happen when in vehicles. The lag doesnt subside and when i go to the main menu and try reloading it crashes. I have seen people talking about some prefetch file meant to fix this issue, but I have the file (SR2_PC.exe) and its still laggy.
There's no fix I'm afraid. PC port of the game is in a terrible state: it can lag, it can crash, it can drop frames and everyone deals with the problems. I suggest you to wait for the patch but... well... we don't know when it comes (I bet next year)