Erratic fps problem

[Blazer]

I have a strange problem with fps. It seems I am taking an 11.1fps hit. I know that it is exactly 11.1 because I have vsync enabled and while flying against a very low demanding scene I only get 48.9 fps instead of 60.0 fps. This problem dos not show up every time, but most. I have tested it many times and it is a 11.1 hit across the board. If the scene normally produced 35fps I get 23.9.

One time I start RF4.5 and get 59.8-60.1 fps flying against the clear sky the next time I get 48.9 fps, no other changes made to the system?

Any ideas?