Have you changed the Overvoltage proterction relay? It is among other things responsible for idle control. It is the little silver box to the left of your fuse box. It has a red top with a 10 amp fuse in it. Check the fuse first - dealer gave me a replacement for free, and if that doesn't work replace the relay, it simply pulls out and plugs in. Cost me about $120 Canadian and it is the best repair/replacement I have made.
1995 E420 - retired
1986 420SEL - retired