The old guy at the radiator shop was talking about the same kind of thing.
His point was that many times the wrong cap was being used, therefore allowing the pressure and fluid to escape too easily, and since there was no return system in place, the radiator would suck up air to return back to the radiator, and over time, presto, overheating from lack of radiator fluid.
His solution was to use the correct cap for the car, and go to the newer system of a resevoir to prevent air being sucked back in.
