2008 F150 air temp fluctuates it does great on full cold and on full hot, will not keep a constant temp while dial turned to medium temp. Is this a blend door problem?

 

I was having similar problems. Only my heat is completely gone now. Just went to a shop for diagnostics and they want $83 an hour for at least 2 hours to fully diagnose it. Called a mechanic buddy out of state for advice and he thinks it is the actuator for the Blend Door. Hopefully it's not expensive.