Coleman FAU mo# L8T04012UHA11B, NG. The main fan stays on after running a heat cycle and continues to blow cold air. Won't stop until power shut off to reset. This happens intermittently. Then it cycles OK, until it decides a few days later to repeat this glitch. Is there a possible problem with the internal thermostat for the furnace that is telling the fan to stay on? Or possible just condensation on circuit panel?