CC amp is under the instrument panel.
CC actuator is under the AFM
Hall sensor is behind the speedo on the instrument cluster.

The system relies on the earthing through the stop light bulbs so if you have earthing problems, bulb problems or wrong wattage bulbs these will have an effect.
The actuator arms may need lubrication. In the worst case scenario the actuating arms could seize causing the actuator motor to short out or a high current to go through the circuit which will burn out your CC amp.
The CC amps - as with other electrical units in the eighties Benzes has a circuit board in it that has been known to develop dry joints ( or cold joints) If you are handy with a soldering iron you can run over the circuit board and re-solder any suss solder connections.

In short many problems with the cruise control.
This is another of those problems where just throwing parts at it may come expensive exercise.

Your Indie dealer should have the appropriate test equipment to sort out problems.