not really sure to be honest, I had been downloading some films/tv programmes like 24 and a cple of months ago i could no longer use the standard port number, all i did was change to a different one & it went back to normal - i usually get full speed depending on what site i'm using.
currently d/l Lost season 1 but only getting around 1meg speed from my 4 meg conn, i put it down to using a public torrent site for this d/l where people dont share as much.