The thermostat is a good place to start.
Test the freezing point of your antifreeze.
Be sure the radiator is full, don't trust the overflow bottle, there may have been, or maybe still is air trapped in the cooling system.
Make sure the AC condenser or whatever cooler might be in front of the radiator isn't interfering with the air flow through the radiator. bent fins or dirt accumulation in the fins will cause your problem. Gene