[Dale fan]

There are more things that could be wrong. Blocked exhaust (bad cat or muffler), fan clutch or even a bad radiator cap or blocked radiator. The gauge could be bad or even your "new" thermostat, I had that happen to me on a Autozone special. Something else would be the gasket on the water pump was put on incorrectly or slipped during installion blocking the water ports. Had that happen once too. Pull the thermostat out totally and see what happens.