Had a problem like that on my 84 500SEL a few years ago. I found there was a bad connection at the idle controller. I found the problem when I removed the mounting screw that holds the controller in place. Pulled it up so I could see the connector (engine still running at the time) and the idle went back down. I held the controller and moved the cable up and down and side to side and the idle would jump up and down. It turned out to be a bad connection in one or more of the contacts in the plug. After trying to clean the contacts without success, I carefully removed the cover for the plug and removed the wires one at a time and squeezed contact with pliers VERY VERY lightly, just enough to close up the gap. You will see gap I am talking about. Just be careful and don't get the wires mixed up. Make yourself a chart of pin number to wire color before you start removing the wires. I haven't had a problem of high idle since.
I have had a problem of LOW idle, but not high idle. That turned out to be the idle valve.

Paul

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84 500 SEL (307,xxx miles)