Hi,that could happen if the thermostat slips lower, out of the groove. That might not be your problem, I'm just saying. Hopefully the block is nice and smooth. If the thermostat housing is not true, or just for the heck of it, I would change it. We sometimes used to sand a housing with emery paper on a sheet of glass to true them. Usually the silicone and gasket is enough. I silicone the new gasket to the housing to hold the thermostat firmly in place. and let it set before bolting the thermostat housing to the block, with more silicone between the gasket and the block. Good luck