ASE asked for me to help you.
I suggest you hook a cooling system pressure tester to the system and drive the vehicle in the city and monitor the pressure in the system off and on. Without side temps of 50-90 the pressure should be no where close to 16psi. Now drive it on the highway and check it a few times. Especially when the temp gauge starts to go up. High pressures may now be visible. If so then I suspect a possible concern with the head gaskets leaking and allowing combustion heat and pressure to get into the system.
Another possibility is trash has collected in the coolers in front of the radiator causing reduced air flow to the radiator. Another possibility is an oil sludge concern causeing engine resistance and causeing excessive heat. I do recommend you make sure the heat is gettin as high as it shows by checking the thermostat housing with a thermal gun.