I had this exact problem when i got my 300CE. The indy replaced the entire air-flow meter/POT assembly with a good used one and the car's been running perfectly ever since.

I should also mention that as an experiment, i tried putting another POT onto the good AFM a few months after. I followed a few posts on here about calibration etc, and i got exactly the same high idle problems. By moving the POT around on the side of the air-flow meter, the idle speed is changed dramatically. I just put the original POT back on and problem went away, normal idle.

Since this high idle was a problem the previous owner never sorted out, i suspect it's why his transmission also gave up and had to be replaced - overheats when you're in heavy traffic and the engine is trying to do 1500 RPM. Previous owner spent a fortune trying to sort it out according to the receipts. I'd start there.