I have an older Janitrol Moffat furnace in my garage. It was converted sometime in the past from propane to natural gas. It takes quite awhile for it to heat up enough for the fan to kick in and once it does, it runs only for 45 seconds then the fan quits. It will cycle like this all day if I let it with the burners going full blast but the fan kicking in and out. I have to shut it off after a few hours because it smells like it is overheating. We installed a new control valve and a new high limit switch with no difference noted. Do I need to build a shield to protect the switch from the direct cold air blast from the fan? Do i have to junk this furnace and replace it with a new one?