Chances could be that the nodes your ISP routes you over to the game server coincidentically happen to be overloaded, which can cause massive packet loss and / or very high ping.
It is also important to note that consistent high ping IS NOT automatically your game doing weird stuttering, that is jitter - as in spikes of ping causing packet loss.
So unless you test your connection to the datacenter via Command Prompt using "tracert <IP of server>" or some program to scan your packet loss, you won't know where on your route you are being hit. It might not even be the FFXIV servers at all.
Source: I work in IT and this is how I win my bread.
Addendum:
https://is.xivup.com/adv
Here a list of FFXIV server IPs, including lobby servers.