Domic,
Engine temperature is controlled in a few places - most notably - how is the fan working for you? Does it blow at all at a high speed when the temperature of the engine is high?
Also, make sure after the thermostat was installed that all the air was bled out of the cooling system.
You could have a radiator flow issue. You could have a water pump circulation issue. You could have a base engine problem at the cylinder heads.
So - some testing is in order to figure out what's going on. Good luck,
-Dave