The HVAC box has an electric actuator motor on top of the box. The gears inside crack and slip on the axle for the arm that moves the metal lever that moves the vane inside the heater to adjust the temp blend. Replace the actuator--or take it off, open it after carefully observing how the grey axle is positioned, superglue the white gear to the axle, and reassemble.
You can remove the hush panel under the passenger dash, take off the 6 screws that hold the glovebox door on, and then unclip two wires from the actuator and remove carefully not to lose them two screws. Small hands will help.
You can first test, by watching the movement of the arm while you change the temp from hot to cold with the key ON. You'll see that you can gently move the arm the way it should go to help the actuator with the slipping. You'll also get a temp change if the motor is running while you're doing it and the coolant is hot.