I bought an 18 yr old home with original Bryant furnace... 1993. When the thermostat reaches 1degree below the set value the thermostat clicks on. I can hear the fan start to blow for about 90 sec then shuts off. The heat will try to kick on and runs for maybe 30 sec before shutting off. My husband went down to the crawl space where the furnace is installed and saw the fan stop blowing when the heat kicked on then furnace turn off. We have had several repairmen come look but none seem to be able.to diagnose. We've had the furnace services and the high limit switch replacee.g. We have also changed the filter and ducts cleaned out. Can you help me with what is wrong?