I had the same problem with my 84 500SEL. I did 2 things that helped a lot but was very time consuming. First, I cleaned the condensor with Mag wheel cleaner. (the old type, not the new and improved kind that is "Clear coat safe). I cleaned it until it was shinny bare metal. Next, I removed the blower motor and using a mirror, I looked at the evaporator. It had a lot of leaves and junk packed in there. Using a refregator brush (Wal-Mart has them) I would brush and them vacuum, look, brush and vacuum, look, etc, until I got it as clean as I could. I then blew it out with air as good as I could. The evaporator is about 22 inches long and 7 in high and you can't see it at all without a mirror so this process takes a while.
The other thing that I could have done was remove the evaporator and I am sure that would have been better BUT the labor guide gives about 20 hours to R&R the evaporator so I went with the first process of doing it in the car.
The results were a much better cooling system. By the way, I was using Freeze-12 freon.

Paul

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84 500 SEL (307,xxx miles)