I'm scratching my head over this one and would appreciate some advice. My 04 Ram with the heat on in any fan position blows alternating hot and cold air.

When this first started I replaced the radiator as it was leaking. Still had the problem, so last Sunday I replaced the thermostat. It pegs the temp gauge hot while blowing cold air. Then, after a few seconds on hot, it starts blowing real hot and the temp gauge drops back to normal. Once at normal, it starts blowing cold again until it heats back up pegging the temp gauge and blowing cold. it repeats this process. I suppose the new thermostat could be bad, but is there anything else I'm missing? Because the hot/cold air coming out of the vents seems tied to the temp gauge, I'm thinking it's a thermostat issue and not a relay. Thoughts?

I replaced the blower motor and resistor back in the summer when the A/C was acting up.

Thx!
Randy