Heater issues

I recently purchased an '02 Legacy Outback 3.0, 151,000 miles. I was losing coolant & it was found that I needed a new radiator, the thermostat was also replaced with an OEM therm. This was thought to fix the issue I was having with the heat inside the car, wrong.
WHen I start off with the car I get plenty of heat, I start with it out at 80, warms up really well then I turn the temp back down to 72-74 and it then cools down too much, I then have to turn the heat back up to 80 or higher to get warm air again. Plenty of heat out of the floor setting, the cool air almost always comes out of the defrost.
Yesterday I set the temp at 80, fan on 2, floor/defrost setting, I left it there not changing anything, after 40-50 minutes it started to blow cooler air, not cold but cool. I find I am constantly having to adjust the temp to make it comfortable. Someone suggested blend door.
We checked the radiator, fluid was to the top but the overflow was at low. I recently had the oil changed, they said it looked fine, they checked the coolant levels & both were ok, that was about 2 weeks ago. HELP!

AC compressor might be no good, or the belt might need to be changed. the car uses the compressor for faster heat and defrost, without it, it takes longer and is not as powerful, vice versa with cold air

I turned the heat down this morning & it still got really warm in the car, my drive was only 30 minutes so didn't cool down. I am wondering if it could be the thermostat that regulates the temp inside the car? The AC compressor, the car heats up very nicely at first only after travelling a bit does it cool down, could that still be the compressor?
Thanks for your thoughts.

ya, it can be a bad belt, i would take a look at the belt

Ok, I will have my mechanic take a look, thanks for your input!

yup, update us once you get an answer!