There is a firmware update 3/03/2008 for that router.
/Support/Technical Support/Routers/WRT54G
Linksys, A division of Cisco Systems, Inc.

Product: WRT54G
Classification: Firmware Release History
Firmware Date: Jan 09, 2008
Release Date: Mar 03, 2008

Last Firmware Version: 1.02.5

Make sure yours is up to date. If you still have a problem, then it may be time to replace it.
More info
Linksys WRT54G series - Wikipedia, the free encyclopedia