I an not an expert enough on these things to say that the amp or the actuator is to blame for sure. This sounds like an amp problem, but I am a linguist, not an electrician.
Acording to what Peter told me, the amp is good for about 10K, after which it won't hold the speed steady, and the car will acelerate and decelerate when the CC is engaged, The amp is usually the cause of problems, but there is something in the actuator that can send a surge back to the amp if it is defective, which will damade the amp, so he suggests that both devices be sent back to make sure both work well together.
Rebuilt amps cost less and may be superior to new ones, as transistors and such are more troublefree than they used to be. Peter says he has gotten none back once he has heard that the owner got the CC working. He guarantees them for 4 years and the actuator for one year.
Thomaspin, on this board, mentions another rebuilder who I think he sais charges less.
GDI has a website where he mentions a lot about these gizmos.
Semibodacious Transmogrifications a Specialty
1990 300D 2.5 Turbo sedan 171K (Rudolf)
1985 300D Turbo TD Wagon 219K (Remuda)
"Time flies like and arrow, yet fruit flies like a banana"