Here's what I was able to find out about cruise control problems:
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, if the wrong light bulb is used for a tail light, it can also cause the intermittent cruise control symptom.
Here is something for you to read:
http://www.mbca.org/pages/tech/MBCA_Troubleshooting_cruise_cont.htm