I wrote this about a month ago:
http://forum.square-enix.com/ffxiv/t...-at-Peak-Hours
By the looks of it, the problem is reaching a lot of AT&T customers. I strongly recommend you follow the steps to run a traceroute, and see which hops are causing the issue. Then use
http://www.whois.net/ in order to find out who/what that hop belongs to. Make a complaint to the proper department, call SE support and make them aware of the situation. Chances are, as has been stated before, it is one of the middlemen between your ISP and SE's server that is causing the issue. They think they can get away with it because most customers are ignorant. Show them you aren't.