Heater working intermittently

I have a 2000 Blazer 4 door. My wife drives this vehicle. She said for the last few days on her way to work, the heater blows hot air shortly after the truck warms up, but about 10 minutes later, it starts blowing cold air? The fluid level is fine. If it matters, the radiator was replaced about 3 months ago. Could this be a thermostat problem?

Could be a problem with the blend door under the dash.

could they have used non-dexcool antifreeze when they replaced the radiator? causing a blockage in a heater hose?

Auto or manual AC/Heater controls?

Have you checked the stat for losing engine tempt?

Always pull rad cap when cold and check rad coolant level?

Feel heater both hose hoses for being hot to the touch.