Quote Originally Posted by NTFDAY View Post
Old radiator hoses do have a tendency to collapse under pressure and heat and will restrict water flow.
Along the lines of checking the simple things first, you do have a spring inside the lower radiator hose, right? If not you could be sucking that hose closed as rpm's increase, severely reducing your flow. And when you say you had to get another radiator, that was a new/rebuilt unit and not one from the salvage yard, right?