I have seen many posts of 124's blowing hot air out some vents (usually the two side vents) as the AC blows cool our other vents (like the middle). This can go on for minutes and then go away. Both my 90 and family members 92 300-e do the same. The secret cure is as follows and cost nothing. There is a main ground going from the engine block to the bell housing for the tranny. Should be on the driver side of the bell housing. Clean were it bolts to the engine and the tranny. Not only will your thermostate sensors work better for the cooling system on the car (the triggering of the aux. cooling fans in front of the radiator) but your AC blowing hot air issues will instantly disapear after years ofblowing hot and cold air. I imagine all the main grounds on all the older cars should be scraped clean. I had a 81 300-D 123 body that did the same but I nevcer found the cure. This was an accidental find as I was really out to bring the Aux. Fan Sensors back to life and it cured that too. I never figured out that years of my electrcal problems came from something so simple.