Heater problems 280SEL
 

Hi,
I have a 1984, 280 SEL . The heater puts out cold air when the car is on the road. If I let the car warm up to operating temp the heater works ok, shortly after the car is moving the heater blows cold air. I have checked the solenoid controlled heater valve and the electric circulating pump both work ok. The heater valve energizes and de-energizes when the switches on the climate control are operated. I checked the hoses and heater core and they are not blocked. I think the thermostadt is ok because the car comes up to operating temp and the hoses are warm to the touch. Is there a way for cold air to enter the duct work if maybe a door/flap is sticking? When the switches on the climate control are operated airflow switches from dash to windshield so I think they are working. Any help would be appreciated.
Thanks, Bob