I found this handy writeup yesterday on diagnosing cruise control problems; it includes instructions for soldering the amp circuit board:
http://www.mbca.org/pages/tech/MBCA_Troubleshooting_cruise_cont.htm
Among other things, it states the importance of using only Osram or Bosch bulbs in the brake lights, because they have a special voltage or something. (I take it this is because they are part of the cruise control wiring circuit for some bizarre reason.) So I checked mine last night and I have the wrong bulb in one of my brake lights.
First question: Could this bulb be causing the intermittent function of my cruise? Have I done any permanent harm by trying to use the cruise with this bulb in the car (I've attempted to activate the cruise maybe only 10 times since buying the car).
Second question: Where can I find Osram or Bosch bulbs other than by mail order?