Cool air and or A/C problems

[300ESUPER]

My 1989 300E has an a/c problem! When driving the car I turned on the cool air and it was working fine for about 20 min then suddenly very hot air started to come out of the vents. I turned the air off and turned it on again and the hot air was still there. I went home and turned the car off. About 3 hours later I took the car for a drive and the cool air was back. 20 min later hot air again. What could be causing this problem? I changed the push button controls for the heat and the a/c system on the dash and I cleaned out the small sensor on the roof where the sunroof controls are. All out of ideas.

Please Help

Fab