I own a 86 420SL Type 107 in Germany and see the same problems like you.
The idle is smooth when the engine is started and cold or below its normal working temperature.
When the engine reaches the normal working temperature (85 degrees C) it starts to run rough. After stopping the car or driving very slow (3-5mph) the rough idle is very uncomfortable.
I fixed an intake leakage at the air distribution tubing and checked the ignition, distributor cap, rotor, cables, plugs without success until now. The mixture is checked and in specs. All old plugs were clean and renewed. The exhaust of the car shows black carbonizing from running rich.
I am also looking forward to get some hints from the net to solve this problem.