Heat turns cold

I turn on the heat and it gets quite hot. However, right around when the engine reaches normal driving temperature the heat turns to cold air. Coolant levels are fine. I even checked levels after engine was hot and it didn't drop any. My engine temperature is normal. Thermastat and water pump are fine. It's cold outside and I am at a total loss.

Does the air ever go back from cold to hot again?
If so, there might be air in the coolant system (not sure how you would get it out, though, other than draining and refilling).
Does the engine temperature gauge change when the air goes from hot to cold?
If you have a rear A/C, do you always get hot air from the floor vents in back even when the front goes cold? If so, this would narrow the focus to the front.

Check and make sure vacuum is getting to the blend doors for the heater.

Disconnect the AC compressor. If heat returns it is as rhandwor explained a fault of the blend door.