Is the new radiator aluminum? If so i had a simialr experience with a new aluminum radiator 4 yrs ago, it cooled fine when I first put it in. It ran 165F going down thw road at 50+ MPH or higher and 160 F degreesas soon as I let off the throttle and slowed down to 35 MPH or slower. The next spring it ran 190+ all the time I had filled it with distilled water and Water Wetter and then added antifreeze later for the winter storage   I had it reversed flush this past winter and it is now running 165 F again I did buy and install a sacrifisal anode that screws into where the drain plug goes, maybe that will help Let us know what you find