As Said, replace it, maybe even go 20A. GFCIs are pretty compex and the electronics do fail over time.
IME, it's often caused by something on the load like other outside outlets or lights, as someone else said. More often it's a post light. Too often the direct burial wire gets nicked or cut by landscapers, irrigation, cable, etc and is either unknown, or a shoddy half-assed repair is made. Then, when it rains or whatever, intermittent problems arise.
