Myself and a few friends in Chicago, IL are experiencing this issue with the cable company RCN (
http://rcn.com/chicago/).
The problem has been persistent since late Sunday night (or early Monday morning) of this week. We've been communicating via Google Hangout while we play and notice we experience lag at the exact same time. Interestingly, prior to Sunday night we haven't experienced any issues with lag throughout beta phase 4, early access, and the first six days of release (besides the obvious login issues).
I'm running a ping to what I believe is our server (Cactuar - 199.91.189.31) from work and home. I never lose a packet at work (high speed business class line with a reputable company) but lose an unusual amount from home. Here's a screenshot of pinging the IP ffxiv.exe is connecting to from work (on the left) and home (on the right):
http://i.imgur.com/8yOgVeT.png
I should note that my ping to google.com remains uninterrupted throughout the packet loss experienced at home.
We have yet to raise the issue with our ISP as we were previously convinced it wasn't their problem. We'll be logging a ticket with them tonight if it persists.