The excessive heat 240 degress may have done in your thermostat and it is stuck closed. just a possibility. Still as REGs points out you could have a radiator issue, do a pressure test to rule out leaks, either it is a water flow problem or an air flow problem. Do you have a fan shroud around the radiator ? It is also posible that the radiator could be blocked up from slag and rust from the engine. Do you run a radiator pressure bottle ?