The Linksys WRT54G series are reknown problematic routers. Their firmware has problems & Linksys never fully fixed the problem. They can be quite unreliable in UPnP mode, better port forwarded. Check for firmware updates all the same.

Someone found a fix, however is not recommended for everyone to try since playing around with firmware could render your router inoperable if mistakes made; http://www.gnutellaforums.com/196843-post8.html

BTW when you did the ShareAza port test, did you have ShareAza open? Best to have it open at that time of doing the test.

Oh .. and also btw ... Norton & Windows firewall are reknown firewall clashers. They should not be both activated at same time. This results in 'less security' not more, plus other problematic issues.