Linksys wrt54g is a "lemon" in the respect the firmware has bug problems that Linksys never bothered to fully fix. UPnP is simply totally unreliable & (unless you use a 3rd party fix for the firmware that is available) the other better & more reliable option is to Port Forward the device (better than UPnP at least) which will provide more reliability when used for p2p, games or any other heavy bandwidth use.

1. First & most important step is the first one, setting up a static ip: Static ip instructions for Mac OSX Click on blue link) ...... bonus Answer

Linksys.com - Support/Technical Support/Choose a Product/Routers and Access Points/Wireless Routers/WRT54G

2. Port Forwarding for the Linksys WRT54G