You measured the radiator inlet and outlet first. That's a good sign. Did you measure the radiator across the rows? You can often spot a clog when one row is cooler than those on either side.
Do you have any idea what rpm you're running at those speeds? I wonder if your distributor advance is getting pulled too far with higher rpm.
We are brothers and sisters doing time on the planet for better or worse. I'll take the better, if you don't mind. - Stu Harmon
