W124 cruise control surge - vacuum?
 

Are there any vacuum components in the cruise control system on an '89 300E? I bought the car with 94,500 miles on it and within 1500 miles the cruise control amp gave out, so I had it replaced. (I hear it's an 'every 100,000 miles kind of thing') Now, with 182,000 miles, the car is surging a lot when the cruise control is engaged.
Is there a vacuum component in the cruise control system that may be leaking? Or, am I looking at having to replace (or re-solder) the cruise control amp?
I had it in the shop for a major service (180,000 mile interval) just before this began. Should I mention it to them?

THANKS ALL!

Ours surges in unison with fluttering speedo needle, which appears needing lubrication or other maintenance. This is when temps are cool--it eventually warms up and needle and cruise both become steady.

I believe yours has the updated motor servo, which replaced the vacuum unit around 1984/85(guessing?), hence it is unlikely you have a vacuum leak causing the surging. If the replacement control unit was used, ie. not new or rebuilt, my guess is that it has just reached the point where a re-soldering is needed.

A bad regulator/alternator brushes that produces voltage surges can in turn make the cruise speed surge. Simple voltmeter test for steady charging voltage at cruising speed will rule out this rare cause.