I had a very similar experience on my 1986 420 SEL....began to remove the actuators and see if any diaphragms were bad.....before that I removed the CCU...and after a few minutes found the three solder joints on the printed circuit board that hold the temperature adjustment pot were all loose!!!!

Resoldered, put back into the car - NO MORE PROBLEM!!!!

Still working--and the air comes from the correct vents and temperature holds steady!!!

Good Luck!!
Mark Pavelka