From the description you provided (very accurate BTW) I suspect an electronic problem in the CC module. It sounds like the uphill load is causing the rpm to drop faster than the engine speed servo can be forced to correct, in other words an electrical limit is reached and there is a trip point or safeguard that tells it to drop the cruise to "off". I would first want to swap another module, than if that does not fix it I would swap another speedometer into the instrument cluster, that is where the road speed is generated by a rotating magnet, at least in the cruise situations I have worked on. I have never heard of this problem before. I send modules to General Development Labs for repair and they can, if you request, add an adjustable potentiometer to further fine tune the response and prevent surges or drifting. They might even test your module and advise.
http://gdl-online.com/begin.html
The system you have is overall working so its not that anything is "dead" and you are better off than having a completely non functional cruise. I have used cruise at freeway speeds and I find it a great way to move your legs around, a welcome thing on long trips but I've never had any downhill dips at speeds like that and I usually kick it off manually when I am doing over about 75 MPH. When my cruise worked on the '87 300D I set it at 100 MPH one day and it held fine
I have seen CC drop out when I passed under high tension lines, the 150+ kV ones, and attribute that to poor shielding. MB had a note about using cruise in police cars (what, Mercedes police cars? not around here!) anyway it said something about rf getting into the module and it would kick off, at least the default is to off, it would have been a nightmare if they turned "on" with some passing police car making a radio transmission
