This may have been covered before on this registry.  My '86 Coupe began "surging" particularly at lower speeds (35-50mph) with the cruise on.  I discovered that the cable going from the cruise servo to the throttle had stretched a little in the 25 years I have owned the car.  It just required a tightening of the cable.  It is described in the Haynes or shop manuals.

Did that correct the surging issue?

Yes.  I posted it because I wanted to save C4 owners the trouble of going to a repair shop for a simple fix.

thanks for the tip. mine was doing it from a vacuum leak. glad it's fixed.

Adjusting the cable solved most of the surging problem, but I still noticed it at certain speeds.  I don't recall that it did it when new.  You mentioned that you had a vacuum leak.  Could you tell me where exactly the leak was?  I would like to do some further checking.

the previous owner taped up the vacuum hose on the right side of engine that goes from the throttle body to the valve cover. i found it by accident. the cruise control will vary the speed usually within a mile of set speed. if it is a little bit that maybe normal. as far as a vacuum leak, check all your vacuum hoses. could be they need to be replaced.