Atqb12

Hi I have a 2006 f150 with the 4.6 and im having a heater problem that I can't figure out.. The heater will sometimes work and sometimes not for the most part it blows warm.. I have changed the thermostat, checked the coolant, I have flushed the system, and the heater core separately, I changed the blend door actuator. I have no clue what is wrong with it, for a while it worked if I would start the heater on the blower setting on 1 or 2 it would get hot then I could change the speed to high.. Any ideas thank you