Been having some similar issues.

A couple days (8 Nov) ago I was getting lag spikes and it felt like there was a pattern so I started writing down the time they occurred and it turned out to be every 28-29 minutes like clockwork. Usually it was just a minute or two of unresponsiveness but 3 of my 13 data points were disconnects with 90002 error. On the 13th lag spike since I started noting the times, I got the 90002 disconnect, and then when I tried to relog there was a long unresponsive loading time followed by another disconnect that was 10.1k or 101k something, don't recall exactly. After that I didn't have any repetitive lag spikes for a couple days.

Now today I am getting regular lag spikes again, this time at 20-minute intervals like clockwork, again some are just a moment of unresponsiveness and some are 90002 disconnects.

I at least appreciate that it's predictable when the lag spikes are going to hit after I determine the timing pattern of the day. I can time my crafting in between them with little fear of being disconnected and losing my mats.

Still sucks though, and it's only FFXIV that becomes unresponsive during these lag spikes, any other network activity is not affected so I don't think it's an issue on my end.