Pool pump and circuit keep tripping breaker

[Jesse Ferrara]

first year experiencing this problem, pool pump will run for about 3 hours at most than will trip the afci gfci 20a breaker. If I reset breaker it trips instantly.  If I unplug the pool pump and reset breaker it still trips under zero load.  The feed is a 12g ran underground presumably in conduit to a manual Westinghouse disconnect and connects to the pump with an older style 3 prong twist lock connector. If I leave it over night off ,the next day it will work again for up to 3 hours when I turn it on.  What the hell is going on??  Being a lineman by trade I understand electric but residential isn't something I'm very versed in. Any help would be greatly appreciated. Thanks in advance