Intermittent electrical issues are tough and can be frustrating. If you are not comfortable with wiring call an electrician. If you are capable then you may want to try the following to help isolate the issue.
I would disconnect the wiring at the spa (first turn off the breaker at the disconnect) get a GFCI receptacle and temporarily connect pigtails from the spa wires to the GFCI receptacle. Understand you are only connecting ONE of the hot wires at a time with the neutral & ground to the GFCI receptacle. Cap the other Hot wire, don't leave it connected to the spa. Plug in a hair dryer or heat gun and let it run to see if it trips. Repeat the test with the other hot wire. If one or the other trips the GFCI receptacle then check the wiring carefully, that's probably where the problem lies. If the disconnect GFCI trips but not the receptacle then it could be the GFCI breaker in the disconnect, try a replacement GFCI breaker. If neither trips while running the hair dryer or heat gun then it points to the spa.