I had my G5 connected to a wireless router for the longest time, then switched to using a cable again when we built a house. The ethernet port was off and on...very frustrating. It wasn't detected, then it worked great, then it gave me a very narrow connection, then it was inactive...so I bought a USB adapter.
Then lightning ran through my cable modem and network, taking out the adapter and the rest of my ports. USB and ethernet down, FireWire to go. (funny how that still worked...)
So I had CompUSA do an estimate and the logic board was in trouble. $1100 to repair, so I bought an intel iMac.
I say all that to say it might be the logic board, as was my case, because this sounds very familiar.