I had a similar promlem with unstable idle on my 300CE and the problem was the idle microswitch.
Remove the air cleaner completely, top and bottom to reveal the throttle linkage. Make sure all the linkage and the switch are clean , mine was covered in oily dirt , then check that the microswitch clicks every time the gas pedal is up.
You may need to bend the switch arm very slightly to make it work every time
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Larry