I don't know what you will pay for the brk you are going to use to replace the one you think might be defective but you might think about renting a Multi Amp Model SR-76 Relay Test Set. It's a bit of over kill but you can determine if the breaker is tripping early and how early. Breakers wear and begin to trip early, usually because they have been burdened an excessive number of times or are used in place of a snap switch to turn the circuit on and off under load.
If the Brk isn't the problem you could install a (Drantz 626) line disturbance analyzer to determine what the voltage and current characteristics are when the brk trips.
