I encountered the same problem in my 92 400E. I took my car in for its 90k service and my mechaninc found that I had a blown fuse for my front engine fans and was approx. 1/2lb low on refrigerant. Both of the items were fixed and my car is blowing cold now, whether I am standing still or going 90 mph. Believe me, it made the difference in the Maryland heat and humidity.
[This message has been edited by mcelroyb (edited 08-14-2000).]