Many thanks to the collective wisdom of the list.
After owning the '90 124 for almost a year, I got it running correctly for the first time.

All stalling, idle hunting, low idle (etc, etc.) issues were cured by replacing the Air Flow Sensor /Potentiometer, and the huge flat spot (r.e. Huge Flat Spot thread) was cured by turning the adjustor on the EHA up a 1/4 turn.

This after falling prey to the 'Benz Boutique' mentality where the Master Tech sez "You should consider budgeting $500 for us to start tracking down the problem"..

I had just bought the car, and didn't have time for protracted diagnosis.
I had already exhausted my bag of tricks for finding vacuum leaks, and didn't 'know' the 124 f.i. system yet. I authorized them for $250, and they couldn't find anything.

Imagine my consternation to read here that I could disconnect the computer by unplugging the EHA, and the car would (and did) run great.

Don't you think information as basic as that would have one of the first tests be to disconnect the EHA?
NOOOO! The first test should be the full-on $150 vacuum leak via smoke test.

When I suggested that my initial research indicated that there may be some form of potentiometer involved, the answer was 'that was waay to esoteric', and besides, he had never seen one fail.
From what I learned here, both of these conditions are typical failure modes for the 124.

No doubt someone will leap to the defense of the independent Master Tech, but after having been one from the early '70s to early '80s, there's no excuse for jerking a customer around with that crap after they've had 15 years to get familiar with the technology and typical failure modes.
rrrrr.

Its a fantastic machine, I can see that now, though too rich for my blood.
Sorry for the rant mode.
Again, many thanks to all who have posted the good stuff.
Mark

After years of frustration, this forum has turned me into a "half mechanic" with all the tool at my disposal. My wife is pretty happy that I can solve most of her car's problem and all the routine maintenance. When I work on cars, she rarely nag any more, unlike when I spend my time in front of computer.

Congratulations.