I have the same problem in my 1993 190E 2.6(48,000 miles). With the car running, but stopped, the air will not get cool. When I am moving, say driving down the highway, the air gets cold and stays that way until I have to stop. After I stop, the air will turn much warmer. Needless to say, that is not a good thing living in Dallas.
Also, the higher I turn up the fan, the warmer the air gets no matter what speed I am traveling.