1986 300E aircondition

[04 Diesel]

I had problems with my ac last summer and dont want to go through that again. The ac worked sometimes and sometimes it did not. When it was 80 or 85 it worked good, but any hotter out it was bad. Also when I first got in the car and started to drive it worked good then if I had to stop at a stop light it would stop working. The car was switched to 134, so where would be a good place for me to start? Any ideas would be great. Thanks.