Just want to share something with this unindentified network issue that i too have when connecting directly via ethernet to my WRT54G.
The last time i used a physical port connection to my router for internet access was when i bought my Router, and that was around 5 years ago. I cannot even remember if it ever worked, cos as soon as i connected the router to my modem (as soon as i bought it), i use wireless.
Anyway, this unindentified network issue i just found out that is is NOT relating to firmware of router, NOT relating to modem, NOT relating to Windows 7. It had to do with MAC address clone. There is a tab in the Router CP that deals with this and mine was always set to "Disable".
What i did was set it to "Enable", put in the Local Area Network (LAN)'s MAC address (ipconfig /all shows MACs) and click Clone Your PC's MAC. Then Save Setttings to save.
After this restart all devices, router, modem and machine.
And that was it.
Not sure if this will work for you but it has for me.