It might be your modem, some of them have an internal firewall. I can't find any info about whether yours has a firewall or not. But...your model isn't listed on the portforward page, normally modems that suffer these kind of probs are listed there.

You could try changing ports. There's a list of alternative ports here, follow LOTR's advice about how to set up.
http://www.gnutellaforums.com/showth...274#post189274