The Stovebolt Site Forums The Parking Lot Big Bolts strange overheating problem

As Stuart suggests, it might be possible that you developed an air lock at the thermostat. A mechanic friend I respect routinely drills a small hole in the plate of every thermostat he puts in any engine which ensures that you avoid getting air trapped on one side of the thermostat. As cheap as thermostats are, you might consider putting a new one in. You can always test your old one using a pot of water on the stove -- just watch to see if it opens as the water heats up towards the boiling point. Best of luck.