Your ISP might be seeing your connection as a Peer to Peer connection (BitTorrent, and such). If their filters are misinterpreting your connection as such, they will cut your connection. A VPN will bypass such filtering, but unfortunately causes additional latency. I believe these detection systems are automated, so low-level tech support representatives will likely know nothing about them. See the following post:

http://forum.square-enix.com/ffxiv/t...P2P-throttling

I hope this is somehow helpful, as I really don't know what else to suggest.