Thank you for the reply Sheepona.

Is any part of your PC overclocked or does your GPU have a factory overclock? Is any part of your PC overheating?

Set compatibility mode to Windows 7. Right-click on the ffxivboot.exe file and select ‘Properties.’ Then select the ‘Compatibility’ tab and choose Windows 7.

This may not be an issue because you've already tried a clean boot, but make sure that any anti-virus software you have has the appropriate exceptions for Final Fantasy XIV.

We hope we can be of assistance in helping you enjoy the exciting world of Final Fantasy XIV!