Hi Artello.
What you are experiencing is
Z-Fighting. Since FFXI runs on Direct3D8, there is no solid solution to fix it. It's a common thing when people play the old game at higher resolutions than what its engine is capable of displaying. There are several things to try.
You might want to set mipmap to 4 but it won't solve 100% though. Change your graphics high or low. Rollback your Nvidia driver version to 375.70 and see if it can solve. However, reducing Z-fighting is not known. It is only based on your own hardware.
Some people like me learned to live with it.
You may want to search for Direct3D8 to Direct3D9 Proxy and see if it can fix your problems even it can be done with vanilla. I doubt we are allowed to talk about any wrapper/third party tool on this official forum though.
Sometimes I wonder what your CPU is because AMD and Nvidia aren't friendly toward each other but you can try this thread to FFXIAH.