I think your instinct is right in stating that the centrifugal switch is probably not releasing, thereby tripping the breaker.

My money is on the centrifugal switch mechanism, which a motor repair shop would have to repair.

- MTR

Just a follow-up,

If the issue here is directly related to the tank pressure (which it seems to be) it makes me feel that head pressure may be causing the problem.

You mention you do have a new pressure switch with unloader, so just to confirm, the pressure bleeds off through the unloader when the unit shuts down at maximum pressure, yes?

Try this: run the unit until it shuts down and quickly disconnect the power and attempt to turn the pump manually. If you cannot turn the pump or if it is very difficult to turn then head pressure is probably building up causing resistance. If the pump turns easily then the motor is probably the issue, probably the starter switch.

- MTR