A slight uneven idle roughness seems to be the nature of the beast. It happened to both my '84 190E 2.3 and 190E 2.6 at about 40K miles. It is probably more noticeable on manual transmission cars as the slight load of the torque converter on an auto. trans appears to somewhat damp out the roughness, and the load of the A/C compressor will mitigate idle roughness on a manual transimssion car.

I believe it is related to a slight uneven cylinder to cylinder fuel flow that develops in the KE system, which also increases emissions and often places these engines on the ragged edge of passing field emission tests. You can run a can or two of injector cleaner through the fuel system, which might help. Also make sure you are using the OE type NON-RESISTOR spark plugs. Many replacement plugs are resistor type, and they may effect idle ignition energy enough to contribute to both the roughness and higher emissions.

I severe cases of this idle roughness you can actually feel/hear misfires at the tailpipe, and if you run an emission test in this condition it will probably bust the HC limit.

Duke