I think I have found the problem. It's a "Network Privacy Tool" called Portmaster. The studders appeared to be correlated with a spike in Network activity, so I shut down portmaster and now I can't make the studders happen after significant testing where studders were happening frequently before. Why Portmaster should cause this effect only for FFXIV after Dawnbreaker is not clear. And why Portmaster was not effecting the other games is also not clear. But I was not getting much if any benefit from using portmaster anyway. So that's why nobody else is seeing the problem. I'm guessing Portmaster is not a very popular tool.

Tomorrow when folks wake up I'll run some dungeons and confirm that the studders are gone.

Thanks for all your comments. I'm glad to find out that the issue is not with FFXIV.