And of course the radiator cooling fins are not plugged up from years of accumulated junk. It could be as simple as an air pocket trapped in the cooling system, but if its been hot this many times, now the head gaskets are in question. A compression test/ leak down test is probably in order at this time. If the AC works well, you could be freezing the coolant in the heater core, depending on how much water has been added.

After the head gaskets:

These days, being a new thermostat doesn't mean it is good, they are cheap, replace it again, or test the last one installed. Wash out the cooling fins on the radiator as best you can Junk really collects between the AC condenser and the radiator.. Make sure the cooling fan functions. Then make sure the cooling system is actually full and doesn't have a trapped air pocket. Have the correct coolant freeze level protection.