the radiator is only one part of the system, the rest are the passages in the engine, the pump, the metal pipes and the thermostat, you may have a squeaky clean radiator but it wont cool down the engine if your engine has deposits on its cooling galleries more than 1mm thick, similarly if the coolant intake pipe is rusted out, coolant will not move at the required rate to and from the radiator which will also cause overheating.
You need a whole system acid flush - this may cause massive leaks if your cooling system looks like "zungistan", that doesnt mean that the acid treatment was bad for you, it means that the acid just cleaned off the rust that was blocking holes in the system which could have let go anytime soon.
just recently I did a citric acid flush on my mercedes benz, although it had used coolant all its life there was still some silt and rust in it, A very mild flush of 400 grms citric acid to 8 litres (MB procedure btw) of water in the system and then 3 full flushes with plain water removed a giant amount of rust and scale in the system. After that the engine temperature stays very stable at 90 to 95 degrees centigrade, before that it used to hit 100 or even 105.
btw citric acid is available at your freindly parchoon ki dukaan.