If the radiator and cooling system is fine as you claim it to be then the only thing left to check is the exhaust system.
A blocked catalytic converter can cause such problems.
When the exhaust pipe is blocked, the hot air and gases coming out of the combustion chamber stays within the exhaust system, dramatically hiking the operating temperature in the process.
That's just physics, my friend so get it inspected as well. I'm quite sure it'll solve your problem.
Hope it helps.