Are you equipped to check some electrical parameters (voltage, current, resistance) ?

Check coolant temp sensor. If this car's temp sensor is like my 87, 260E, then when the engine is cold, you should get between 2.5 and 3.5K resistance acorss the sensor terminals. When the engine is at 80 degC, the resistance should drop to between 250 and 350 ohms.

EHA and idle control valve have been mentioned previously. Though EHA is less of a factor once the car has started and idling.

Another item is the OVP relay, when did you last check/replace it ? It supplies voltage to the idle control valve.

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Saumil S. Patel