I have a 97 F 150 4.6 liter I'm having coolant issues.I have changed thermostat water pump on the engine but I'm still getting a lot of pressure and its blowing antifreeze out the cap.if you let the truck run for about 15 minutes or so before you head out in it the system works fine its just got me confused and I just hope somebody had this problem and can help me out.I've also change the radiator cap

 

Sounds like a blown head gasket.