here are some hints:
1. the needle on the pressure tester should not move - you should have not trouble keeping that thing at 16 pounds or so for an extended period of time...
2. check your fan clutch. maybe it's bad
3. could be a bad temperature sensor giving a false reading
btw, i don't get that air-trapped-in-the-system thing... the entire system circulates and the upper hose is above the thermostat... how could air possibly get stuck in the system?