overheat due to lack of circulation and heat exchange at the radiator. However the thermo may be restricted from fully opened position. The only way to check it is to pull it and check per the Service Manual. But before that check the radiator once warmed after you shut off engine and feel with hand palm on the inside of the core and look for cool areas. If operating correctly the radiator surface should be a relatively uniform temp that indicates that all the core tubes are passing coolant and no restrictions.