Re: what causing this in crysis ?
Maybe it is triple buffering being off. Means that if you have vsync enabled and your monitor does 60Hz for current resolution then if your PC cant maintain 60fps (59fps or lower) it locks to 30fps unti lit can maintain 60fps. And so it will lock itself down to 20fps, 15fps if previous framerate target cant be maintained.
Solution is to either use D3DOverrider to force triple buffering or when your in game menu alt+tab and then get back to game as atleast with ATI GPUs this will make triple buffering get enabled.
So what triple buffering does is allow you to have any framerate between 0 and 60fps without sudden jumps to lower framerates.