One more overheating problem

Just because you have a 15 psi radiator cap doesn't mean that you will have 15 psi throughout the entire cooling system. When a pump is working good, the suction pressure of the pump is significantly lower than the discharge pressure. If the lower hose is weak, it will collapse the hose even with a tight system because the pump is creating a vacuum at the suction of the pump. That is why Chrysler put a spring in the lower radiator hose, to prevent that very problem.
Rod