How are you verifing the engine temp? I would suspect one of 2 things. possibly it is not overheating, the temp sensor is causing faulty gauge readings, in which case change the sensor. If it is overheating check to be sure the exhaust sytem is not blocked, possibly bad catalytic converter.
water pump
Either the water pump impeller is loose on it's shaft, or you have a leaking head gasket.
more than likely you need a new radiator for it, when you install it back flush the engine.
Check your electric cooling fan
Check the water pump
check your fan clutch if its weak it will overheat
Bad thermostat maybe?
is your car equipped with a coolant overflow bottle replace thermostat have radd removed flushed and rodded
Water pump defective.
Have you flushed the heater core?
When you changed the therm, did you make sure you bled the housing. This is done by the bleeder valve (looks like a brake bleader) on top of the therm housing, and a second one on top of the engine. If you get air trapped behind the thermostat, it won't open and the engine overheats.
My guess is the head gasket, have same problem. Nightmare