A good radiator shop can give you a better diagnosis. They can put a pressure gauge on the radiator to see if you have a leak or if the head gasket is leaking. They'll also check the radiator cap to make sure it's holding the correct pressure.
Sometimes the radiator cores get clogged and back flushing isn't enough to remove the gunk. You might need to get the radiator re-cored or replaced. An easy way to check it is to get one of the 15 dollar IR temperature guns and check the temperature of different parts of the radiator. If there are cool spots on the radiator it indicates that it's clogged.