It's an ongoing issue guys... SE has had an active investigation open since March 5th. We've had several maintenances attempting to address things on their end that they've been able to identify. Tonight's is I think their 3rd one specifically for such stability problems identified as being on their end. Otherwise, there may be issues in your routing at play as well that may be compounding it. If so, you should see signs of congestion (highly erratic response times, possibly occasional timeouts at the same hop) between you and the last few hops if you run some tracerts to the server your client is actually using (the ones starting with 199 are the servers in Canada--not the 184 one posted all over the internet... that is a web server and not a game server).
http://forum.square-enix.com/ffxiv/t...f-%28Mar.-5%29
If you are still having issues after this maintenance, in addition to posting details as requested in the March 5th post linked there, you may want to investigate your routing to see if there is something between you and SE's ISP (Ormuco) that your ISP can investigate and address.
Issues in between you and SE's ISP are the responsibility of your ISP and/or their routing partner to address--not SE. Your ISP has the means to pursue issues occurring prior to Ormuco (SE's ISP) on your behalf, as they have the service agreements with these third parties to carry your data beyond their local network segments. Without such agreements in place, you would only be able to access content hosted on your ISP's network--forget anyone that uses someone else for internet access. You couldn't even email someone who uses a different ISP. This is something that is enforceable by your ISP and their service agreements--SE has little to no influence over problems that may reside on Level3, TATA, Cogentco, etc. segments, but your ISP does if they are a routing partner.