The theory is that coolant was flowing through the radiator too fast to adequately cool.
That theory is completely at odds with accepted thermodynamics.
Generally, overheating at highway speeds is water flow related.
But you don't claim to be over heating, just running "hotter",.
You you have numbers?
Highway speeds generate more heat in the engine, so what might be "ok" at idle and no load fails at a higher load. Your recored radiator means nothing on the surface, was it flow tested? What core was put in? Airflow out of the engine compartment can be an issue as well. Air can't go thru the radiator if it can't get out of the engine compartment.
