Running hot on the highway, but cool in traffic could be that the engine bay is restricting the egress of hot air at speed or cooling air is going around the radiator at speed due to the configuration of the frontal components on the car or the bottom radiator hose is collapsing due to lack of an internal spring to keep it round. It could be a myriad of goofy things too, like the water pump impeller spinning on the drive shaft at higher engine speeds.
I would first check the bottom radiator hose for a spring. Then I might make some spacers to space the rear of the hood up off the cowl by 1 1/2 to 2 inches and see what happens at speed. That will tell you if the air has been damming up in the engine bay. Take a look through this wiki article and see if anything jumps out at you, in addition to what I've said.