Tim Greif - West of Chicago
There's no way furnace thermocouples can go bad that often. You may have a weak detector in your gas valve, to the point where it fails to respond to the millivolt output of your thermocouple. You could also have a weak pilot that's not fully heating the thermocouple. Either will cause the furnace to be intermittent. Most thermocouples put out 15-25 millivolts when heated with even a BIC lighter. Most gas valve will register the pilot ok with less than that. On my old Reznor unit heater in my shop (manufactured in 1964), every few years I need to remove and blow out the pilot burner jet assembly with compressed air or I get the same intermittent operation. The flame isn't strong enough to keep the thermocouple covered, and the gas valve drops out when idle. Just a thought.