That was the first thing we thought of since intermittent. Not sure what to do about it though. Get another thermostat? It was cold. Can that cause it to stick? It was a Stant and I put the hole at the top. It happens once every 5-8 months.
Stant use to be the number one quality but not anymore.
Unfortunately, the only way to test it is to pull it and heat it up in a pot of boiling water. But then you can watch it to see if it opens each time.