When it starts to overheat, try running a garden hose on/through the radiator and see what temp it cools the engine down to... If it cools down a significant amount, you are either not getting enough airflow through the radiator or not enough water flow through the radiator may need a recore... (assuming that the engine side of the cooling system is good - water pump etc...) A recore does not recondition a radiator, it just cleans it out...
The engine temp should run where the thermostat is rated...
If it's a timing issue, try advancing the timing a bit and make sure that it doesn't spark knock/ping... Retarded timing may cause it to run hot...
