Why does my lamp post light trip the breaker after a couple of hours?

Outside lamp post light. I have replaced the eye senor, the light fixture, the 15 am in the breaker box, light stays on for a few hours then trips the breaker. It is not connected to a GFCI. The 15amp breaker only has this light on it.. Wiring is buried in the ground & is not disturbed by anything around the wiring. Light has been working for 28 years, old light fixture could have gotten water in it so I replaced everything when it originally went out which was several weeks ago. Breaker did trip with no light attached.