I've got exactly the same problem on my '95 E220.

Just got mine a month ago & still trying to find out the cause.

I just checked the electrical connectors/plugs and found that one of the plugs on the coolant temperarture sensor was a bit corroded & cleaned it. I drove it for awhile and did not show up. I'll find out tomorrow if that was it. I doubt but let's see. I'll let you know.

I also checked the vacuum hoses for leaks and they all seem to be OK. Idle changes a bit when I pull one out one at a time and goes back to normal when plugged back.

In case it shows up again, where is the actuator on the M111?

John, how much was the actuator anyway? I suppose this can be DIY thing?

Anymore advice on what's the culprit?