I have seen this on MB v8 cars, and usually they need a little fine tuning.

I would have someone put the car on a scope and see what is going on, the likely culprit are bad ignition wire ends(too much resistance) and/or partly clogged CIS injectors, assuming that the valve guides are not worn, letting excess air into the fuel mixture. A vacuum guage will help to diagnose a valvetrain problem.

If you don't have someone with a scope, I would look real hard at the secondary ignition components(esp. the plug wire ends and proper non- resistor plugs), and also consider replacing the injectors. They run around $30.00 each, and after twenty years, the spray pattern is probably poor. The fuel mixture will have to be reset after replacing the injectors, though I would use a can of BG44K first if it was my car.

I knew someone who bought a 500SEL new in 1985, after three years it also had a funny idle. The factory trained tech blamed the fuel injectors, and he was right. It turns out the cheap regular unleaded his wife used caused the injectors to gum up, a can of BG44K fixed it.

I would make sure the ignition is right first, and then move on to the rest.