Cold start over heats on 2003 Ford Explorer Sport Trac

I've replaced a leaky hose and thermostat. The car is idling rough. When I start the car in the morning the temp gauge will rise to almost the top,then it cools down and stays at the proper temp since I changed the thermostat, but its still doing this. Pressure test came back okay, but the pressure from the last time it did this blew the reserve tank cap off. I have no idea what is causing this. No smoke coming out of the top so I don't think it's a head gasket issue. What could this be?

Asked by for the 2003 Ford Explorer Sport Trac
if sure not head gaskets, may have air trapped in system. get to a shop have it checked.