This could be something to look into:
Model:
1987-1988 Mercedes 300E, 300D, 300TD, 190D, 190E
Symptom:
Cruise control will disengage intermittently. Engine cuts out when vehicle comes to a stop.
Beginning with the production date of May 1987, the Hall-effect sensor, which is mounted on the speedometer, may have a defective cold solder joint on the sensor output pin.
If your vehicle exhibits these symptoms;
1: Cruise control will not engage at all.
2: Engaged cruise control will disengage intermittently.
3: Your 1988 vehicle cuts out when you come to a stop but will restart immediately.
Gently wiggle the output pin. If any movement is detected, the sensor is defective and should be replaced. After June 1988 an improved part was made (stamped 6.88) even though the part number remains the same.
Also wrong wattage bulbs in the tail lights may have also be the culprit. Some light bulbs fit and look the same as our OEM MB bulbs, but are actually different and will cause the cruise control to not function properly.
I had an aftermarket bulb that caused my issues and when I replaced it with an oem, the cruise worked fine.
Take a look at this thread also:
Cruise Control Not Working