Running for 3 months fine than suddenly trips breakers is a big point. You say your flying blind, but, you manage to locate a run capacitor. Is there a start capacitor installed also? Most single phase 120vac motors that employ a run capacitor also have a start capacitor. The start will be the larger if the two and is switched out once the motor reaches speed. If you are saying the unit starts, runs, and will produce 2000 psi then trip the breaker, then, while a run capacitor could be the problem, it could also be the motor itself is failing. A motor will run without a run capacitor but would eventually overheat. You could monitor current draw, but, the reality is the cheap 110vac motors are not known for being reliable or rugged. Working for 3 months filling bottles and guns may be all you got from her. You could lookup how to size a run capacitor and pay the $10 for one but don't be surprised if the motor itself is the issue. Good luck with DIAG and let us know what u find.