I don't know what it is, but thermostat's have been dropping like flies this season. I've never seen so many thermostat problems going through my shop that has gone through this year. And to top it all off my thermostat is sticking open for the first time ever. I'm not saying you defiently have a thermostat problem, but i'd change it anyway due to the thermostat curse of the 2000s. I'm serious this is going down in history. The bible was wrong it wasn't the end of the world for man kind it's the end of the world for cooling systems.