Running Hot

It seems as though your cooling system has more than one problem going on at the same time. The fact that it heats up with less air flow and cools down when it's on the highway shows there is an airflow problem through the radiator. The fact that there is no pressure in the system indicates the system is not tight. Your cap might be relieving the pressure. Use a coolant system pressure tester to make sure your system can hold 16 psi. Do you have a catch tank that is vented to atmosphere connected to the cap vent port? This could be where the pressure is bleeding to. Also to check your t'stat and to make sure it opens at 180, remove it, put it in a pot of water and heat it to 180. you should see it open if the water in the pot is shallow enough. I've seen brand new t'stats work incorrectly. Hope it helps.