1991 Mercedes Benz 300SE 3.0 I6, 113,000 miles. The car idles fairly smooth, but when I put it in gear the idle becomes quite rough. I have performed maintenance of all filters, plugs, and oil change. I have not changed the plug wires (they appear to be original) the plugs are not discolored and seem to be firing correctly when checked with a timing light. The wires are expensive so I would prefer not to change them if it is not needed. The car runs well at speed, but I am afraid the roughness is being masked by the increased Rpms. This problem is not always present, but I have been unable to determine a pattern. We have been using 93-octane gas as recommended. We recently purchased this car so do not know the history of this problem.
Do a compression check first. The plug wires can be checked by observing the running engine in very low light "night time" and watch for arcing. If they are arcing, replace them and your problem should be solved.