Our first suggestion would be to backup the router configurations to your PC. Reset the router back to factory defaults. Reapply the current firmware for the version 1.0 and 1.1 router, 1.1.13. Reset the router back to factory defaults again.
For the first test, reload your configurations from backup and see if the problem persists. If the problem persists, reset the router back to factory defaults once again and reconfigure the router from scratch. Add one level of complexity to the router until the problem comes back. Once the issue returns, we will have a starting point as to what is causing issues.
One question I have. Is the DHCP server turned on or off of the WRT54GS? Is the router set up as a DHCP relay?