Make sure the electric fans are pushing air thru the radiator towards the engine. My 190e 2.6 was running a bit hot. One of the fans was replaced and was wired backwards. It was pulling air from the engine compartment. A simple test would be to place a sheet of paper on the electric fan grille and see if it sticks to it,if it does its working right.
Also you should checkthe operation of the thermostat by boiling it in a pot of water and examine when it opens and if it stays open. I had a similar problem and was puzzled that the car would heat up rapidly in traffic but would run cool on the highway. I discovered by boiling the thermostat, that it was warped and would only open a crack thus poor circulation at idle. My cooling system now works perfectly.
1990 190E 2.6
1996 Grand Voyager 3.3
1985 Mustang GT 5.0 5 SPD
1982 Suzuki GS 750T