take the thermostat out completely and test it, the one you have fitted could also be faulty highly unlikely but it does happen, atleast that way the thermostat is 100% ruled out, as for it still overheating, id take the rad off and stick a hose down it and check flow is coming out as it should, failing that take out your heater matrix and run a hose through that checking its working correctly, failing that pull all the hoses out and hose them, failing that id say you may have a crack in the head/block possibly? and have you checked your oil level? overheating and high temperature readings can be caused by lack of oil in the engine, if none of that works then all i can say is its something on the wiring side or just a sensor that needs changing