My Armstrong 80 furnace will work fine for about 10-20 minutes then turns off before reaching desired temperature. Example: My new thermostat reads 62 but is set for 70 degrees. When furnace kicks on, it only stays on for about 10-20 minutes and increases room temp to 65. I can then toggle the thermostat so that this furnace kicks back on. I already cleaned the flame sensor which didn't seem to make a difference.