Frustrating Overheating Engine

I have a 97 Mercury Mystique 4 Cyl. 2.0L. The engine started to overheat. So I replaced the tstat and the temperature sensor that sits on top of the tstat including the wiring harness. The engine continued to overheat. I then replaced the radiator and the engine was still over heating. I then replaced the upper/lower radiator hoses and the head gasket/resurfaced the head. In addition I replaced the water pump and freeze plug because the freeze plug was leaking coolant. Since replacing all of above said parts the engine is still overheating. I have bled the system of air. I have made sure the tstat opens properly. And I've flushed the coolant system. The engine is still overheating however it does not lose pressure and there is no leaks. Side note the AC/Heater are both functional. Please please please help!