Hot tub trips breaker after running for a while, but it doesn't do it all the time:

Check the amp draw on the breaker. Could just be a weak breaker.

I recently had this happen to me. I found out it was the heating element for the hot tub. It was about 10 years old and finally gave out. It was a relatively easy fix and while I had it out I replaced the pressure switch just in case! After doing this I didn't have any more problems. Hope this helps.