300E a/c blows hot and cold

[mugsy]

My 1990 300E has icy cold a/c, but suddenly it's started blowing hot air after the a/c has been on for awhile. I've been looking for a pattern to this, because sometimes I can drive for 20 minutes or so with nice cool air, and other times the a/c blows hot right away. With the temp. select wheel set to 65, fans on auto, and climate control button set to EC, the fans seem to run really fast, regardless. If the normal button is depressed, the fans quiet down. Changing the temp. select wheel doesn't seem to have any effect.

I've taken it to a shop thinking that it just needed more freon, and they gave it back saying the freon level is fine, and they didn't have a clue why it was blowing hot air. I noticed that when it blows hot, it's sometimes a lot hotter than the outside temp, as if the heater is kicking in after the car has cooled down. I confess to not being sure exactly how the a/c works, so I'm not sure if this is a thermostat issue - like the once the car is cool, the heat's coming on to raise the temp again? Is the a/c & heat dependent upon or reacting to the outside temp , or only inside temp? Any suggestions appreciated.