heat does not work at low RPM's

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Our heater will blow cold air when vehicle is at idle RPMs or low speeds. At highway speeds and higher RPM's the heater works fine. The air direction damper valve will switch from upper to lower vents and rear heat works at higher RPM's as well but not low RPM's. The inside temperature controls will adjust correctly. We changed the thermostat and it did not make any difference. It has the 3.5 V6. XLT model.