i have 1981 ford t-bird 5.0 L, 4 spd.auto loses heat.

is all the air out of the system? maybe check for leaks or check thermostat also check to see heater hoses are hot going into the fire wall and hot coming out...

You might have a problem with the damper door opening then closing up. An '81 T-bird should have a vacuum operated door so you might have a vacuum leak. The heater control valve is operated by vacuum. It should be located up under the hood where your heater hoses are. This opens and closes the valve so radiator fluid flows or stops going into the heater core. The damper door opens and closes depending on if you have the air cond. on or the heat. This works so both aren't open at the same time. Check both heater hoses, they both will be hot to touch if the heater valve is open and the heater core itself isn't plugged up. The damper door will be open when the air cond. is on and closed when the heat is on. If there is a vacuum leak going to the door valve, the door will open during a vacuum drop (usually under acceleration)and will close again when vacuum comes back (under idle conditions) You'll need a vacuum meter to check vacuum) good luck, Kenny/MrBlueOval

please reread; I said, both hose are always hot, no low coolant every, no heater control valve on this old car, now its a blend door for cold to hot air.

You only said heater "hose" is hot, not heater "hoses" or "both" obviously the "blend door" isn't working. Either way it should be vacuum operated and you got a vacuum leak. That's a 33 year old car and I can't remember that far back whether it was actually vacuum or electrical in that year Bird, probably vacuum but your blend door isn't working properly and that's what is causing hot then cold then hot again air coming out. Try running it only on defrost and see if that makes a difference...

Sparky222 you may have something , bet thats a small air hose up to a coil style sensor up near top of dash