My car had the same issues and were resolved by replacing the oxygen sensor. You state that the car idles fine when cold (oxygen sensor running in open loop until heated) but after the car warms up it will start to stumble (oxygen sensor in closed loop).

I believe I read in one of your earlier posts in this thread that the car will stumble unless you go WOT (again oxygen sensor running in open loop).

The wiring for the oxygen sensor enters the vehicle cabin right in front of the passenger seat. Lift up the mats on the passenger side, move the seat all the way back and look for the connector there.

Unplug the connector while the car is acting up and see if that clears up the stumble.

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Sam
1989 Mercedes 300E