Overheating at 60mph. Do I need a new thermostat, new radiator, or both?

I recently did a coolant flush on my 1990 NA, so the coolant should be fine.

I was out doing a little highway driving when I realized that at 60-70 mph in 5th gear, the temperature needle was very close to the "H", so I put it in neutral and cruised to the next light. By the time I got to the light, it was halfway between the H and the center. However, as I started from the light and ran through the gears back up to 5th, it went back to normal, then started to creep back up to overheating.

The only thing I can think would cause this is a bad thermostat, but I'm honestly at a loss and have no ideas. Any advice would be greatly appreciated, since this is my daily driver.