Ross Kirkland
A roofer would be required to have a look to advise. If its only occasional, then the leak likely only happens when the level in the gutter gets to a certain height due to the large volume of water. This could be a blocked downpipe causing slow drainage, or the leadwork/felt/ membrane that forms the box gutter may be ripped or inadequately sealed upstream (above the height of low water volume). There are various sealants and methods, some of which can be DIY'd, the skill is knowing which product. material or method to use. I would post the job in the job section and pay an expert to at least diagnose, if not repair. Hope this helps