I'm not sure that this is your problem but it could be. I see that you're windows is set up to use half of your installed RAM to use as shared video memory for your GPU. IDK if this is still true, but back in the old days, when you allocated this via BIOS, any shared system memory was strictly reserved for your GPU, not allowing windows to do anything else with it. If this still holds true with windows 10, then this would suggest that you've only got 4GB useable RAM for all other windows resources, which barely meets minimum.

As a 960 card is only really good for 1080p, there's hardly a situation in where games will need more than the 2GB dedicated memory, and any shared system memory needs to be re-allocated to the system as a whole.

Also, you say you've stressed tested everything right? So what programs did you use for this purpose? Also, does the FFXIV still crash, say when your PC has been off for a long period of time? Any recent upgrades? Tried yanking sticks of memory only to leave one in at a time and then try to play FFXIV? You could easily have a bad stick of RAM and wouldn't know it until something tried to access it, meaning some things will run completely fine if not accessing the bad memory module.