I'm not sure if this is you problem, but I had an AC/heat flip flopping problem. If I turned on the AC, it would blow cold, then hot, and back and forth. When I put the heat on, it would work if I set it hot, but it would come on strong then shut off or cool the car if in median settings. The culprit was a bad thermistor. It was hard to trouble shoot because at the temperature it was most often when I'd test it, it would read fine. However, if it cooled just a enough, it would drop from like 1700ohms to 2 and that would tell the AC servo the car was way cold, crank up the heat. When it jumped up to the higher ohms, it would tell it its too hot cool it down. My car is a 79 and your's and 80, for the 40 bucks you might just replace it.

Frank.

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'82 300SD