Compression tested ok. Any ideas?

04 Ford freestar 3.9L over 65 mph overheating, never at idle. Just temp gauge run high let off gas, temp drops fast. New thermostat, temp sensor, overflow tank & cap. No mix on trans. And or oil.
Pump seems to be working new belt no leaks noticed to eye around engine or in cab thinking heater core. Water pump has small 8th in. Less play side to side on pump shaft. Oh yea compression tested ok. Started with finding cap cracked watched and its seems to bebacking up then cap relievs pressure antifreeze in tire rim wet uder overflow tank no rad. Cap just overflow for fill. Any ideas?