Have shop check for exhaust gases in the coolant, (bad gasket, crack in head or block) leaks gasses into water and heats it up, if okay, have shop check temp of the radiator (digi thermo) top of radiator and bottom, should be much cooler at bottom (like 15 degrees, if not, radiator bad. (Iagree with Rocky 1234)
Chances are its the RAD, they do go bad, fins just don't distro the heat, most overlooked problem, and a relatively cheap fix. Hope this helps.
(pesumed you have checked, timing, plug heat range, thermostat, bad hoses collapse when hot, radiator cap, flow of coolant when up to temp, water pump)