I replied to your initial post the other day and somehow it didn't make it. So, I'll try again.
After the breaker trips and the motor is stopped, does it feel excessively hot? Also do you smell anything like overheated insulation or burnt wiring? That could indicate a shorted turn or turns on the motor windings. I doubt it though. Could be a breaker that has gone bad too. When the pump is running listen at the power panel for a buzz or some noise from the breaker and when it does trip does it feel warm? Is it a GFCI type breaker? That would be more prone to failure. If it's a regular breaker then possibly the contacts have gotten crummy and developed a high resistance and it's heating up. They will trip on overcurrent or thermally if the load slightly exceeds the rating. Changing the breaker is a relatively cheap try. Sometimes the mechanism will just go bad and they'll trip below their rating. Since you said the pressure is right and it's pumping OK you can rule out any problem with the motors starting circuit.
Let's get comments on my suggestions and we'll go further.
Al