Truck heater heats up when its not moving but when driving it blows cold air

What is your question?

98 dodge ram 2500 Hearter problems and it's been for some time like a year done every thing to try to fix it bought new fan motor checked all the holes temp is good it just won't stay hot when I drive works when I am not driving tho controls seem to work ok. Heed some input on this please thanks