If it's only happening in FFXIV then it's probably not a hardware issue. You will first want to backup you HDD and then perform a clean install. Or easier buy a new HDD and do a clean install on that disk and assign you current install drive as a secondary drive. This would easily allow you to copy files over to the new HDD and if necciasry you can still boot into your original install by simply assigning you old HDD ad the primary drive and the new one as secondary.

There is no possible way for us to determine what's the problem. Perhaps you should take it to someone to look at it and fix it for you. It's extremely unlikely you are going to find the cause and solution on internet forums. You will be given a lot of of incorrect advice about power supplies or over heating or demon possession.