I have the same damn problem in my 92 190E 2.6 with 97K on it. When the car is moving the temp is fine and when it stops the temp goes up. When I put it in neutral and revup the engine the temp comes back down. Guess what ? I put a new thermostat, new fan clutch and flushed the radiator with a garden hose at home. It still does the same thing. The auxiliary fan seems to kick in at the right temp. Regardless of the fan coming, the temp does not come down until I revvup the engine or until the car is moving again. So temporarily, I added water wetter (Redline) and it seems to help, atleast it keeps the temp below 105-110 in stop & go traffic. I am thinking either there is a blockage in the radiator or the waterpump is faulty. I am kinda stumped because the waterpump does not leak. Anyway this is my 10c worth and hope this helps you.
92 190e 2.6 97K