2001 F150 heater problem

My 2001 F150 5.4L heater does not work all the time. When I am stopped or at slow speeds it works but once I'm over thirty or so it blows cool. My AC works and my engine temp is good. My radiator fluid is also good. When it does work it blows nice and hot.

sounds like you have an airlock in the heater core. you can usually fix it by turning on your engine and take of the radiator cap. let it warm up and then squeeze the radiator hoses repeatingly while constantly filling up the coolant
reservoir