bingo.....you need to keep the coolant in the radiator long enough to cool it....If it doesn't stay in the radiator long enough, it doesn't matter what size radiator you have , how many cores, aluminum, etc.....
It is about 3 degrees for every lb of pressure.....so if you have a 7 lb. cap you can add 21 degrees to 212 to get your boiling point...So about 233 degrees....