Hmmm.... Are you absolutely sure it is the FPS, or could it be the stuttering effect of the drawn vertices? They are switching directions every few frames, and that might appear to be the frame rate.

I'm not doubting the possibility of this, but I just want to eliminate this explanation before moving forward.

Thanks for trying it by the way!

I have an Nvidia GTX 770. I will put in a frame rate counter, so I can see more clearly what's happening.