Just a thought,Before doing any other work i would check the rads are fitted correctly,as mentioned some rads have baffles or other water diversion fitment inside the radiator to ensure the flow arrives at the top of the radiator,this can be established by reading the install information or before fitting just look through 2 top or bottom of the connections and if you can see straight through then there could be no baffle etc and the radiator relies on the pipe connections to get the recommended temperature through the radiator.