PowerMac G4 Won't Connect to Internet

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I have a Dual 1GHz PowerPC G4 connected via ethernet to a cable modem through the Apple Airport base station which connects to an Asante 5-port wired router. The G4 doesn't have an Airport card and doesn't support Airport Extreme, so I am tethered to the wall for now. There is also a 2x2.66 GHz Dual-Core Intel Xeon G5 connected to the internet in this same way. I am using this machine right now so I know the internet connection works. A wireless iBook G4 also connects to the network perfectly. Until recently, the PowerPC also worked fine.

Last week I loaded Leopard onto all of these machines via the Update installation. The PowerPC ran sluggish afterwards, but still connected to the network ok. Then I tried using VPN to connect with my office which uses F5 Firepass. I discovered that the version the office uses didn't support Leopard. When attempting to connect, the VPN window typically pops up and shows the connection progress. This time, it displayed a message saying that I wasn't connected to the Internet. I've restarted the modem, base station and router, restarted the machine several times and gone through the Network Diagnostics several times. I even backed everything up and reinstalled Leopard using Erase and Install. Still no connection. I have connected the Ethernet cable directly to the modem, and also directly to the base station. Nothing seems to work.

When I open Safari, I get the "You are not connected to the Internet" message and the Network Diagnostics button. Clicking on that, in the left margin there is a Network Status listing showing the Ethernet with a red or green light. This will toggle back and forth between connected and not connected every few seconds. Network Settings is shown with a yellow light. ISP, Internet, and Server all show a red light with a "Failed" message.

Sorry for the long description, but I wanted to be thorough. I am at the point where I need to give up or buy a new computer. For what I do the machine is useless without the Internet.

Thanks for any advice.