I went through a series of trouble shooting on my w124 wagon. I had erratic heat. First thing i did was to check the vacuum solenoids that control the dampners. Check the vacuum at the y connector in the engine compartment on the driver side below the wiper. It is a check valve y fitting. Check the rubber to make sure it is not cracked. One line goes to a vacuum chamber in the front passenger side wheel well, the other goes to the vacuum valve behind the passenger side air bag. If all is good there. Check the vacuum solenoids in the car. Easy to get once the passenger airbag is removed. There is a 10mm bolt that is accessed from below the dash. You will need a long extension. I did not unplug the air bag, just put it on a box to keep it elevated. If you changed the sample air blower, you already know how to do this. Remove the brackets that hold the airbag in place so you can get access to the vacuum solenoids. They are all color coded, but take a picture just in case. Pull them off and use a mity vac and pull vacuum and see that they all hold. Check the rubber connectors to make sure they are not cracked. If all is good. Check the heater core temp sensor located behind the push button controller. Mine was pulled out of the heater box and was reading the temp in the dash instead of the heater core. Hope this helps. That was my problem.

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