When I turn my heat on say 70deg, my furnace kicks on and heats the house to that temp. When the furnace kicks off and the temperature goes below 70deg it will not kick back on. I can raise the temp on the thermostat to 90 deg and it still does not come on. If I go to the gas furnace manually shut it off then turn it on it starts up runs to the set temperature or so then goes off and does not come back on. What could be the problem?