300SD Heating Problem, HELP (I'm Cold)

[SirNik84]

I have a problem with my heater in my 1984 300SD. With the climate control knob at max I push the defrost button, or the front vent button, or any settings to make it blow, it blows cold air. OK that’s bad. But it some times blows hot air. It will blow hot air when the car is at idle (when it’s warming up in the morning, or at a stop light for a few min) it will blow hot air, but as soon as the car starts moving it blows cold air again. I don't understand why. Any one knows what the problem could be?

thanks

Nik