Do you guys play in windowed mode or full screen? I have been playing in windowed mode since release and I get those errors frequently... most of the time, it is when I go to another window (alt+tab), but sometimes it happens without changing windows at all. I will admit that it doesn't happen nearly as often as you are describing, however.

It may be some sort of compatibility issue with the new drivers since I know the 600~ series is a comparatively new set. As for Diadumenos... I don't know what your problem would be. Because you are using a 300~ series card, I am assuming it is several years old. This would give Nvidia more than enough time to tweak the compatibility with FFXIV.