I had the same problem in my car (86 560SEL)....... If you that you have good vacuum/ vacuum components, and the cold air comes out the defrost all or most of the time, I changed out the ACC PB panel. I was told that these were high failure items and it was my last resort before i took it to a shop. I ended up buying a rebuilt one for $155 and now she works like a freezer and blows cold through the proper vents.
1986 560 SEL