Profiler, What does "Overhead" represent?

![alt text][1]

When I start my game, FPS if fine but after a few seconds, there's a consistent frame drop, as if something new starts running, however, the profiler only states "Overhead". Theres also framerate spikes every couple seconds, I've tried doing some optimizations to avoid GC kicking in but the problem remains.

Any help would be greatly appreciated. [1]: http://i.imgur.com/frTAU.png