...Hall effect sensor. It sets the idle speed based upon some math formula (don't ask me to explain it any further). When it is defective, the sensor can cause symptoms similar to the ones that you have described.
It is a tiny part that attaches to the back of the speedo in your car. Has anyone pulled the instruments out recently? Sometimes the sensor becomes disconnected when the instrument cluster is pulled out.
Good luck, those are marvelous wagons!
1989 300TE "Alice"
1990 300CE "Sam Spade"
1991 300CE "Beowulf" RIP (06.1991 - 10.10.2007)
1998 E320 "Orson"
2002 C320 Wagon "Molly Fox"
Res non semper sunt quae esse videntur
Not in this weather!