XIV isn't optimized as well as most other games. It hammers the piss out of all subsystems at once, and depending on how your power supply distributes the load it can still become unstable even though collectively it may be rated high enough. Each rail has a max total wattage of output and if your cabling stacks too much on one rail it can starve something like the GPU or CPU aux power connection and cause all kinds of weirdness.

Try moving some devices to other cables if you can to see if it better balances the load across the rails. Might do the trick.