I have a ten year old furnace. The heater's moter clicks on and off, and the blower turns on, but no air, or cold air blows out. Periodically, the heat functions properly. We have head repair men look at it and have gotten everything from the wind blowing too hard to the circut board. I don't feel comfortable paying $375 for a circut board when they aren't sure that that is the problem. Any suggestion?