When I bought my 85 TD the amplifier was completely missing. The PO told me that he had taken it to a shop and they told him a new amp and so forth would cost him over $500, and I suppose they thought he'd bite and kept the old amp for a trade in or something, but who knows?
So I called Peter and he gave me a list of the models that I could use and luckily I got one in a boneyard for $20. Peter said both my actuator and my amp needed rebuilding, so he did this for aronud $325 I think. It's worked perfectly ever since.
I have a 90 300D as well, and from time to time the cruise starts to buck and jump and I just reset it and it works just fine.
I would say that if the used amp works well on your car, then the problem you're having just has to be the amp. I would also suggest that it is entirely possible that the used amp you bought may have another 20 years left on it. Who knows" it might have been replaced the week before the car went to the junkyard or may never have ben used. I'd stick it in and get the other one rebuilt one last time.
There is could be some weak solder joint on the board that passes current perfectly when flat on the bench, but may miss when bolted under the dash.
Good luck in any case.
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"