In my experience phantom GFI tripping is usually caused by moisture issues. It may not be a nick in the jacket, it could be a situation where moisture was introduced to the inside of the outer jacket at either end and has now travelled up the wire and is causing the issue.
Try a gfi breaker instead, as they seem more stable and less likely to trip or no reason. I have always had a disdain for GFI recpticles and usually spec. In a new GFI breaker when I can.