Terrible framerate issues

[Petch1984]

Yeah, it's not even when a lot is going on in the game world, for example I only get a slight dip (maybe 5 frames) when I'm out in the huge ocean - this seems perfectly reasonable as it certainly looks graphically intensive BUT there's occasions indoors, in seemingly simple areas where if I turn around I could drop 20 frames. At first I thought it was light sources but I'm not sure plus it's not triggered by action either, it's just odd. I did have a hour or so near the start where the game ran perfectly, so I guess it could also be a leak. Again, I'd hasten to add it's not my setup, my PC is designed to run editing software and games and nothing else so all background and startup programs are in line, the temperature is sound etc.