Sounds like two separate issues to me.

The miss could easily be an injector —spray, pop pressure, etc.
A while back, I remember reading something about hydraulic knock. Hazy on the details but something to do with a dribbling injector causing delayed ignition and a partially firing cylinder. If you can get your hands on a pop tester, think it’d be worth confirming the state of your injectors.

The RPM thing is weird and points to an issue within the pump, like the governor or some other binding, gunk, etc. Sadly, on most forums the pump is a black box and operating knowledge beyond the basics is rare (I certainly don’t have it). You ran a purge, so less likely the latter. I’d come back to this one after solving the knock.

In addition to Sugarbear’s suggestions, I’d head straight to the injectors.

Let us know what you find.

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1981 240D 4sp manual. Ivory White.