Sounds like maybe some power saving feature kicks in? Did you already try to set windows to "best performance" (or whatever it is called) in the energy settings?
I have a 675mx, no problems at all. Not with 1 hour, 5 or 24 hours.
Isn't it heat? 670mx uses the old Fermi core, not very good with heat control. Nvidea inspector can tell more. Core clock rate, GPU load. If it gets too hot it underclocks itself until it cools down.
It's better to limit framerate to 100 FPS anyway. Overhead is not needed, uses unnecessary power and generates more heat then required. Internal LFS physics also work @100 Hz so its perfectly in sync.