Some variation of the first expression in this section should do what you're looking for:
http://www.motionscript.com/articles/speed-control.html#other-applications
It's designed for time remapping, but it should work fine for rotation.
You'll probably want to increase the value of
mult (10 maybe?), and you may want to keyframe a steady base rotation value so that the expression adds the surges to that (although it would be pretty easy to build that into the expression as well).
Dan