Intermittent, and Runaway Cruise Control -1988 300 SE

[gschira]

Good evening everyone.
The cruise control on my '88 300 SE is acting very strange. Usually won't engage. Sometimes it will engage. When it does, sometimes it works great, but usually it seems to make the engine pulsate or accelerate/ decelerate and not really hold constant speed. Also, sometimes after breaking, it won't resume, and cannot be reset. Sometimes after it is engaged and seemingly holding the desired speed ( and this is the spooky part) it accelerates all by itself. What should I check. What would be causing this?
Thanx in advance for your help............