Carrier model 58mta 9300TS: The furnace will run sometimes it seems like forever without shutting down.. Furnace was installed 03/2004 it just started acting this way lately I have installed a new digital 2 stage thermostat just last week it still acting up... does not want come out of 1st stage of heat anywhere from 45mins. To 4 hours... Any thoughts anybody . Thanks N-BY-NW