I didn't see what your model was--most likely, the surging and the cutting out is a bad CC amp in your case.
To answer your question, the CC amp receives a ground through the brake lights. Bad bulbs, or in the case of my 190E, a loose screw on the tail-light ground wiring can mess things up.
In the case of my SEC, a bad CC amp was causing the surging. Some have said lubricating the throttle linkage was sufficient to eliminate their surging. Hope that helps . . . Cheers, Andrew Seidel.
Cruise- Should I replace the bulbs even if they are working?