72 Ventura heat blows good while setting still but while driving heat turn in to cool air blower is new
You might check the thermostat, or change it out to one with a 180 degree rating. Sometimes when it is very cold out, you can put cardboard or something in FRONT of the radiator to block airflow to reduce the amount of air that gets through. But, I would try the thermostat first. Hope this helps you out.