Running Hot

Ever since I installed the 340 about six weeks ago I have had an overheating issue. A new thermostat ( 180 degree ) rad cap (16 lb) and the larger round top tank radiator were also installed. I have noticed that there is no pressure in the upper hose or rad after driving for 45 minutes. Temperature gauge stays about in the middle on the highway at 65 mph but as soon as I get in the city it keeps climbing. Last night after I got home from the cruise I could squeeze the hose easily and took off the cap with no release of air or water pressure. Tried it again tonight same thing. Anybody have any ideas ? Thanks Bobby