You definitely need to check out and probably replace this section of foam pipe I'm describing. Made all the difference in the world on my '82 300D.
My original symptoms might be slightly different from yours. To get heat, I'd turn up the dial to a higher temp. Out comes lots of strong heat. Then it would cut off and I'd start getting cold air, and it seemed the heat would never come on again. So, I'd bump up the temp dial some more. Same thing over and over until the temp dial was way in the red zone, near maximum.
My theory is that since the heat sensor had no air flow, it was sensing the heat inside the dash which the ducts go through nearby. So the ducts were heating up the inside of the dash hotter and hotter.
With the pipe fixed and the sensor air flow moving again, it seems like the air coming from the vents is just about always comfortable, not really really hot nor cold. And the cabin temp is very stable.