UPDATE

Just so happens that we own a '90 and a '91 300E. The '91 is the one giving trouble.

Today, even though I've read that the ECU rarely goes bad, I decided to swap the ECUs between the cars.

To my amazement, the problems moved to the other car.

So now, the '90 gives the goofy 10% readouts on the DC.

The '91 still requires that I "wake up" the ECU by flooring the gas pedal once for just an instant, but after that it runs fine (it's been this way for three years). Before I do the "floor-it" maneuver, it just sits forever at 50% fixed DC. After I floor it once, it comes to life.

I'm still gonna replace the airflow sensor pot to see if it clears up this original problem.

Wonder where's a good place to get a replacement ECU.........