Congratulations on the fix.
I have been struggling with a very similar problem on my 79 240D with auto, and have eliminated everything except the IP itself. The car has been in my family since new, and was not always like this. Also, other 240Ds are not this bad.
The question I have is how to adjust the governor on the IP? The Mercedes manual gives a description of operation that describes a "breakaway" spring and a "compensating" spring. These seem to be the only adjusments on the governor (except the altitude adjustment, which I have played around with with little success). Is it practical to do this without specialized tools or testers? Am I completely off base? I am not talking about calibrating the pump part, just the governor, which seems like it could be the problem.