[DrRadar]

I am not familiar with the 3.1 but clearly there is a hear dissipation issue in traffic. My first thought is a potential thermostat problem...not stuck closed since cooler on highway but maybe not opening fully. You could remove the thermostat as a test. Another possibility is the pump is not moving enough coolant...not sure how check to that. Or blockage, most likely in the radiator. Gunk can start to block the lower tubes.

Good luck.