Have you been hammered?
Hammered = fake torrent trackers/servers (e.g. RIAA, anti p2p, recording studios, security companies, etc) will keep bombing you once they detect that you're downloading pirated files (which 99.99% are). Sometimes it lasts several days even you've exit your torrent client. This will slow your connection. It's a matter of time until the ISP will detect and ban totally p2p
Try to put PeerGuardian2 and enable encryption in your torrent client (e.g. uTorrent). It should reduce greatly, making it more difficult to ban, and keep your transfer fast (given the file you're downloading has many seeders/leechers, and the port forwarding is not blocked by firewall/proxy).