heating problem
 

1999 F150 4.2l V6 5 Speed Stick 4x4 manual. Coolant level full, temp control far right(heat),fan off/on, flow direction control to floor, panel or def. From cold start to mid range on temp gage heating normal for first 10-15 minutes or 20-25 miles. Air flow/heat from vents slows to almost nothing.If fan/blower is on sound goes hollow gets louder and no forced air from vents. Upon engine cooling down to cold, cycle will repeat. Have changed to heater control module and have observed blend door motor working correctly and when heat is working normal it can be reduced and cut off with the control knob. Could this be a thermostat problem not opening or sticking?