Undulating

Did anyone work out what the solution to DJGIRISH's low frame rate save was? I'm having trouble where after a while the game will drop to around 5fps, and the only thing that will fix it is loading an earlier save, before the framerate drop. Restarting my machine doesn't help (if I load that save). If i start a new game, the frame rate is fine - but that particular save is now low frame rate forever. I'd rather not have to restart (I was only using quicksaves) - anyone got any ideas? The save hasn't got particularly big - it's still in the 5Mb range.