I would recommend taking an extra look at your GPU hardware and see if it's still running at PCIEx16. Tools like GPU-Z can help confirm this if Nvidia's own software can't relay this data. I know AMD's software can. If it happens to be running at a lower rate (and even if its not), take the GPU out. Clean the contacts on the GPU, as well as the PCI slot running a thin slice of paper to help loosen any oxidation from its years sitting there. Re-seat the GPU and see if the problem persists. If it does keep exhibiting a fatal error, there's a high likelyhood you'll need to replace your PSU as GPU really needs consistent power flow for it not to crash under load. It may run fine outside of FF14 but once you put the card under stress it may start acting up with fluctuating power.