tl;dr -- Lots of 90k errors here and an occasional 10105. We fixed it 100% by limiting the graphics to 30 FPS.

Our setup -- My girlfriend and I play the game together on Ultros. I play on my PS4, she plays on my PC. We play side-by-side on the same network, both devices wireless. Like many of you, my computer is far and above the minimum specs for FF14. I have zero problems running any other game, including running Netflix or YouTube on my phone while playing two different games on the PC and PS4. Never had a hiccup in connection.

What we tried first -- We tried all the steps listed in this thread and others, given the problem seems to have been a thing going back over a year at various times on various worlds. This included:
  • Lowering game resolution
  • Port forwarding and triggering
  • White-listing FF14 for Avast virus scanner
  • Killing Avast completely during the game
  • Killing firewalls and adjusting their settings
  • DMZing the PC
  • Trying (but not buying) Pingzapper

None of those worked. Then my girlfriend randomly went in the settings, noticed the frame rate cap, and just set it to 30 FPS. Didn't change anything else -- graphics quality, resolution (720p), or ANYTHING else -- and suddenly she had a perfectly stable connection. Night and day.

Obvious disclaimer -- Maybe this won't work for you. But I was absolutely out of ideas, and we seemed to get lucky. I have no idea why it should work -- my logic says that any graphics issues would be on my computer's end, and shouldn't impact data transfer rates -- but maybe someone more technically saavy can explain it. Either way, we're happy it works now: we went from lagging or random disconnects every few minutes to zero problems with one setting change.

I hope this helps!