P2P and Airport Extreme

I have an internet connection via AR 520 ADSL router. I'd like to use a Gnutella client for file sharing. I have disabled the firewall, both router firewall and system firewall. I have also forwarded the ports, but I'm facing problems because when I start the client (Limewire or Cabos) eveything goes well for 20 seconds and then the connection stopped.
Is this may related with Airport Extreme?
Do you have any suggestion to make Gnutella client work properely?
Thanks