Sounds like the Idle control module to me. rap on that sucker with a screwdriver handle smartly to see if it dies. if the car dies then the solder connectons in the ICM are bad(cold joints). replace it. I had troubles like your on my 380SE and I found that if I rapped on the ICM the car would die. Did it twice to make sure it was me doing it. then replaced the ICM. Fixed it and no more wandering of idle also.

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1986 Euro 500SEC "RUF" 9:1CR, "Rose"
1985 Euro 500SEC Cabriolet AMG Widebody
1982 Euro 500SEL "Blue"
2001 Texas Heeler (Aussie/Queensland X) "Sulphie",
2012 Queensland Red Heeler "Squeak"
Best dogs I've ever had.