I have forced air, and the heat is blowing cold air. The thermostat keeps jumping from 61 - 65 - 61 - 69 and back to 61 with the blower keep blowing cold air. The blower won't blow at all with the thermostat set to auto but when I switch it to on it will turn on and just blow cold air with it set at 75. The pilot light is lit on the boiler and I have hot water. What is causing the cold air? The Thermostat?