Report "Intermittently my gas furnace fan doesn't shut off,so it blows cold air.If you turn down the thermostat until you hear it click,then wait until the fan stops,you can then turn the thermostat back up. And it works again for awhile. so far We have replaced the flame sensor,the thermostat,and now the circuit board. Any Ideas??? The model no.is GNE 075B12A1 seriel no L9840 79850"