I've got a Nordyne M1GH 077A AW that I don't know how old it is. A couple of days ago it was running and running but not heating, no fan. I changed the batteries in the thermostat and it ran properly for one cycle and heated normally, then not cycling again. I installed a new thermostat and the furnace ran a normal heat cycle, but again, only once. What could be wrong and what should I check/change to get this to work properly?