Good afternoon and thank you for posting on the Final Fantasy XIV Technical Support Forums.

I'm assuming you are running the game at the highest setting, correct? A GTX 1060 will suffice if you are trying to run the game at 60 FPS at 1080p, but it can drop in a few areas like you mentioned. Running the game in borderless fullscreen does make the performance of your card a bit less efficient, but as you said, it can happen in fullscreen as well.

It might not affect the frame rate too much, but you can try updating the graphics card drivers (the most recent version is 419.67) and disabling any overlays if they are present.

You mentioned that your GPU has some issues if the temperature goes above 70C, correct? Normally, modern GPUs can go upwards of 90C and still be operable. Is this a normal occurrence with your GPU? I can't tell for certain, but your GPU may be running the game a bit slower because of this. Even if this isn't the case, the frame rates you are describing aren't exactly out of the ordinary.

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