This thread had information on
NVIDIA drivers, possibly consider downgrading:
http://ffxi.allakhazam.com/forum.htm...7244185&page=1
Other than that, normal checks.
Check temperature (read: is fan always on? If yes, probably too hot.) For more accuracy, google SpeedFan to get an exact reading of your GPU, should be no higher than 80 before shit starts to go wrong, though some mobile cards go even higher (90+) before this happens.
Make sure you have updated
DirectX by going to the microsoft website.
If you're POSITIVE it was fine like 6 months ago, do a
System Restore which should revert anything you borked up (too much porn?).
If none of these things work,
Check Files in POL to make sure nothing's corrupted (long shot!) and consider re-installing.
EDIT: Should also mention, try mixing up how you start FFXI up. Does it still lag in true full-screen? With windower? Without? With official windower? If it works fine in one of those but not another, that drastically cuts down any chance its a driver/files/directx issue.