To many answers with no precise recommendation. I had this with comcast then swapped to verison.. Still had it. I finally just got purevpn and fixed everything by using. If you're fed up and on PC then this is about the only real solution I could find. If you're on ps3/4 then the same applies. Forum users will lead you in circles just as much as our ISP/SE. It's either VPN or quit or.. Wait till it finally gets mysteriously fixed.
Edit: this problem affects many ISP's it goes so deep I don't know if anyone knows WHAT exactly causes.
http://forum.square-enix.com/ffxiv/t...and-catches-up is proof enough and so is that sticky topic.