Your assumption on the radiator needing a boil out is correct. Your trouble shooting or diognosing is right on. At idle or slow speeds, the fan and fan clutch is pulling air across the radiator to remove heat from the coolant. At highway speeds, its the ram air that removes the heat. Haven't been a fan of radiator stop leak or flushes, remove the radiator and have a radiator shop repair it, don't forget to tell them about the stop leak additive that you used. One last thought, t-stats set the minimum engine temp not the max temp. The efficience of the system controls maximun engine temps.
Good old Bill