Mrmerlin

For the OP I would also suggest that you replace the thermostat and the rear thermostat seal as well as the front thermostat O ring.
From your info there is a good chance the rear seal isnt sealing.
These parts are in the water bridge, the rear seal is removed by tapping on its side till it is knocked into its center then it can be pulled out, tap the new part in with a socket and extension