I had the same issue with my Savoy and it turned out to be the pulleys weren't running the pump fast enough on the highway. I had underdrive pulleys on it from when I was racing it a lot.I got a set of repro pulleys from 440 Source and "PRESTO" an all stock cooling system with a 5 blade clutch fan cured the problem.Now instead of running 230 on the highway it stays around 190
Gus


64 Plymouth Savoy
493 Indy EZ's by Nick at Compu-Flow
5-Speed Richmond faceplate Liberty box
Dana 60