The Overvoltage Protection Relay can affect idle if it is bad. It is the little silver box with the red top and clear plastic cover about 8 inches left of your fuse box up against the fire wall. Check the 10 amp fuse in it as they often go first. For our cars the overvoltage protetion relay has been upgrades since our cars were new. I also has a rough idle that nothing cured (including almost all new ignition parts from coil to wires, a new fuel distributor and injectors - (all of these helped somehwhat, but OVP relay fixed it all up good. It cost me $120 Canadian with tax, I know they sell it in the FastLane here and you can replace it yourself in about 10 seconds, it just pulls out and plugs in. I used some dielectric grease to prevent corossion on the contacts. This might work, but your best bet is probably to take it to a mechanic and had a thorough diagnosis done.
PS. You can get pretty good shop manuals on CD-ROM from MBUSA
1995 E420 - retired
1986 420SEL - retired