I know very little about mercedes cruise contols. Virtually nothing in fact. It does appear that their cruise controls are not good at dealing with electrical noise from a couple of descriptions here.
How about disabling your alternator output temporarily by removing a wire. Then run car up over seventy and check cruise function.
When I was doing a little electrical work on a 123 I found the ground loops where loaded with alternator noise. It was affecting meter readings. There was even a square pulse efect at idle. I was on the verge of getting the scope out to have a look but did not. There was no point. That may have been the voltage regulator. Maybe this is normal. As rpms increase it is going to be worse with different frequencies generated as well. There is an external capactor on the alternator to deal with some of this. Since the car exhibited no bugs I never pursued it. This amount of noise would not hurt anything simply of a resistive load.
That cruise amplifier is a little more complex though and is an active electronic device. I would not spend any amount of time with this unless neutralising the alternator stopped the problem. Much of this depends on what the speed sensor is generating. A pulse or a variable voltage. I do not have a schematic to examine but if pulse excess alternator noise could get it. Talk about a long shot. I certainly would not take this to the bank. On the otherhand it is so easy to temporarily disable that alternator.