These may be utterly irrelevant to you, but having recently changed my '89 500SE from a spluttering high idle to a 600rpm purring kitten, I can suggest:

• Check the HT cables and system. Put a meter on each ignition cable and check for 1kOhm continuity on each. Clean and if possible tape up any junctions. Examine the running engine in the dark and look for brush discharges and other signs that the high voltage is escaping.
• Clean the distributor cap and look for unusual signs of tracking around the internal contacts.
• Check the connection to the Rotary Idle Valve. The contacts on the connector may be corroded; clean them if so.
• Check the idle jet operation. I found that unbolting it from the engine allows it to be withdrawn far enough to see the nozzle. Using a piece of wire to the battery, you can fire the solenoid, and if it's working a fine fuel spray will squirt from the end.
• Check the idle switch. On mine it's deep in the bowels, attached to the throttle butterfly, but the connection cable is easily accessible. With a continuity meter you should get a contact closure between two of the pins at fully closed, and another at fully open. Verify this.

In my case the idle switch was defective, and one spark-plug cap was open-circuit. Correcting these two, and generally refurbishing everything else in the system, turned this wheezer into a purrer.

Best,

CD