Is it only ffxiv that has this problem? Are you experiencing it in any other games? The usual culprit is due to failing hardware, but it is rare for it to only occur in one game if this is the case. Check several of your games to ensure you see no other corruption with graphics, if you continue to have the issue it is possible you need to either run a file check (if that is an option) or even reinstall the game cause files might be corrupted.