Many ISPs have what are called "file transfer bandwidth limiters." Basically, when they detect that you are using DCC, or a P2P program, or something like that, they make your connection go slow. There are two main reasons for this. One is to encourage users NOT to download mp3s, movies, warez, and other copyrighted material. The other is, when you download a big program, you are using bandwidth. Bandwidth costs your ISP money, they don't want you using more than you pay for. So if you are downloading too fast/too much, they'll slow you down. I'd suggest you contact your ISP and ask them about it.