I recently had a very similar issue and found this thread to be most applicable to my scenario. Pool Light Tripping GFCI - Page 2
Basically, the light would not turn on, so I pulled it up, bought a new bulb, tested it and reinstalled. Then later that night the bulb appeared to work but turned off minutes later, this time I noticed it tripped the GFCI. Again I pulled the light up and made sure the bulb was seated, tested and reinstalled. That night, again the GFCI tripped. I replaced the GFCI, checked the Junction Box and still tripped the GFCI every time I flipped the light switch. It turns out, there was a minute leak where the cable met the bulb fixture, and this was drying out when I was pulling up the light (hence why it would temporarily work then trip the GFCI). I applied some waterproof silicone to this area (after ensuring it was dry and working) and the lights now works as expected.
Moderator comment: This post was separated out from the older thread