1991 Mercedes Benz 300se idles rough in gear

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.