Yea, I missed the part about the radiator being replaced. In the spirit of stretching-it, what about the oil. Many years ago I had a Pontiac (Grand Prix) that would slowly overheat during highway speeds.
A guy at the local parts house suggested that the 10w30 oil was too thin. We added a can of STP oil treatment
and it solved the problem. I later switched to 10W40 oil which had a similar (thickening) effect.
Don