Sounds like a bad head gasket or cracked head.Combustion gases and pressure is building in the cooling system and causing it to superheat.GO to the garage and ask for a "block check" test to be performed on the radiator.A glass bottle of chemical is held over the open cap while the engine is running....if the fluid changes color,hydrocarbons/carbon monoxide is present....meaning combustion gasses in the coolant.Maybe its the lovely Dexcool causing troubles.