Overheating after modifications, what am I overlooking?

I had an different overheat problem years ago. Around town the temp was normal. On the highway it would stay normal until I went

any speed over 60 mph. It would easily climb to 3/4 on the gauge. I finally decided to take the radiator to a shop and see if cleaning would help.

He called me back when he got to it. He had hooked it up to test flow percentage. It flowed only 35%. Only a re-core would fix that.

He put a triple core in it for me. What a difference. Drive as hard as you want and temp stays right at 1/2. Was pretty expensive though.