1902 2.6-AC works well but hot air out of side vents?!

[2manyoldcars]

I have a 1990 190e 2.6. Symptoms: AC works fine out of center vents as designed. With AC OFF, and any “vent” control button pushed in, the side vents blow cold (outside air temp), which suggests that the heater valve is working. I’ll replace it anyway as a preventative measure as it is cheap.

With AC ON, “center vent” button pushed in, and blowing cold air from center vents, the side vents sometimes blow cold, sometimes hot and have actually changed from cold to hot while driving. With AC ON, any button other than “center vent” pushed in, the side vents blow hot. The heater valve seems to be working intermittently. Perhaps the heater valve is not getting vacuum when the AC is on. Would this be in the center control or perhaps the temp wheel or even the MAS? I’ve seen this problem posted on several sites but have not seen many suggestions other than replace the heater valve.
Thanks