I have an airco furnace and was attempting to change the thermostat. I connected the 2 wires to the new unity (Black & White) turned it up beyond the normal household temp. It came on for about 90 secs and shut off. It seems to repeat this process every 20 or 30 minutes but will not keep the burners on. The pilot light is on and the thermo coupler appears to be OK. Possible solution?