Ethernet Problem

I just acquired a dual 1.25 MDD G4. It works great, except I can not get on line. I have changed several cat 5 cables and tried an older 10/100 Asante PCI card. I am using Cable modem (Road Runner). The 867 Mhz G4 works fine on line. (I guess I can assume the cable modem is okay)
Prior to the acquisition of the dual 1.25 Mhz, I had an dual 800 Mhz with the same main port ethernet problem. So I purchased a Sonnet Gigabit PCI card, which after extensive troubleshooting with Sonnet, I had to return because it did not fix the problem.
Now, if I want to make this MDD 1.25 G4 Ethernet operational, I guess the logical thing would be to give another try to the Sonnet PCI card. But, because of the previous experience with the 800 Mhz G4, I am a little skeptical.
Am I overlooking something in Leopard?
Before I invest in another Ethernet PCI card, I would really appreciate some words of wisdom on this problem.
(By the way, the ethernet port is recognized, but it gives a self-assigned IP number)
Thanks for the Help.