Question regarding my 1981 W123 300D heater. What are the symptoms for a bad cabin temp sensor/middle of the dash sensor? Is there a separate fan that draws air over the sensor or does the main fan do that?

My heater works flawlessly for about 10-15 mins. Heat is controllable at high/low mph. Fan starts strong then lowers speed as expected. But after 10-15mins it loses heat and wanders aimlessly mostly on the cool side. Even then it goes hot when defrost is selected or max heat is selected? AC not working, all this is in the EC mode.
Thanks ya'll
Pat