I have the same model modem as you, but i am using a router aswell. You are not using a router.
If you have norton antivirus, then have you configured your port forwarding?
Also, are you downloading a popular torrent? If you are downloading one with a small number of seeds (people sending you the file) it will tend to go slower than other torrents.
Try heading over to
P2P Forums they will be able to help you better with this issue than here.