I must admit that you seem to have changed all the relevant parts of the cooling system. Given that the thermostat is working correctly (ie opening at the correct temperature)it then allows the coolant from the engine block to go into the radiator for cooling. If, as you say, the engine is not overheating, the fan comes on correctly (showing that the circuit is working ok). The only thing I would do next would to be carry out a pressure test on the cooling system, just to make sure there are no leaks at all. From there I would suggest that, given everything is ok, maybe the actual temp gauge itself may be giving a false reading.