heater goes in and out on 2002 Dodge Ram 1500

the fan works great, and so does the heat. The truck runs at regular temp,and doesnt leak anything any where. When I turn the contol knob to heat,most of the time I can hear the door open or close, which ever it does going to heat.But at any given moment the heat just disapears like someone turned the knob to cold. It can stay like that for a few min to an hour or so. I turn the knob back and forth at that point and it doesnt make any differance. When its ready, the heat starts comming out again like a new truck instead of cold air. What do you think is cousing the problem,the door, and if so how do I fix it? Also, this all happens going down the interstate at 70 mph

Asked by for the 2002 Dodge Ram 1500
Sound like a vacuum problem with the blem door id make sure you dont have a leakin one of the vaccum hose uder dash
I have a 2002 dodge ram 1500 had my dash apart twice for heater issues. There are no vacuum hoses in this truck .All vents are control by door electric actuators. The climate control cold/hot has a blend door actuator which operates two doors at the same time changes from cold to hot.I would replace that actuator its possible that it could have a internal short or open making and breaking contact at times.