ie if he is running at 55fps he will drop to 30fps likewise at 100fps he will drop to 60fps
Are you sure that's right? I have vsync enabled, and certainly get "odd" FPS values.

I thought vsync was effectively a "peg" to stop the game bothering to render frames that wouldn't be drawn. (with apologies to those who know what they're talking about for the potential abuse of the words "render" and "draw"!).

That's certainly been my observation.