Some ISPs use packet shaping technology that will throttle BT activity, regardless of port. The technology watches for BitTorrent traffic and then slows it down if it finds it.
See recently ISPs has started believing they have the right to tell you what you can and can't access, or slow you down for accessing something they think is a strain on their network. It's all BS, they just want more money and throttling BT is one way they save big $$$ (less you use, the less they pay even though your bill stays the same).
uTorrent allows you to force encryption (highly recommended) which pretty much negates that packet shaping technology.
My ISP has packet shaping and when I turn encryption off my DLs drop to 10 Kbps, with encryption I get 500 Kbps easy.
