So its on its own GFCI breaker? So the pump should have its own breaker. As a test (if you feel comfortable and familiar around electricity, you could be electrocuted if your not sure what you are doing) shut both pump and light breakers off, you could disconnect the wire going to the "light" breaker and put it to the "pump" breaker(you must disconnect the pump wire and leave it disconnected while trying this. This way you could see if the problem travels...what I mean is, if the problem still occurs, breaker trips, its not the breaker, its the wiring past breaker or light itself. I think GFCI breakers are pricey, so this could be a quick test.

Anthony