1980 300SD 225,000 miles

Cruiz control is intermittent in operation. Usually works for hours but then it seems to loose its memory and the cars slows as the cruiz control is inoperative. Sometimes when cruiz is reapplied the car accelerates with no control until cruiz control is manually shut off. Usually if cruis is not used for a while after the above occurance, it works again for a variable period of time.
Is this a symptom of a bad "amplifier"? Also, where is the "amplifier located"? If I have to replace the "amplifier" where is the best place to get another and are rebuilt units available at reasonable cost?

P E H