Sporadic ethernet link

I had been using the AirPort on my G5 for a long time and have recently moved into a new house, in which I've installed a wired network and have a wiring closet.

Everything works fine except for my iMac. The ethernet connection has been sporadic at best - when it works, it's great, but most of the time since I've had it up the link is simply inactive. This is the onboard ethernet.

I've tried different cables, going straight from the iMac to the switch, straight to the modem...all with no results. The link simply remains inactive no matter what I do. I shut the computer down for about half an hour earlier and brough it back up, the ethernet worked then (and beautifully), but later in the day - without the iMac going to sleep - ethernet was down again.

I've tried looking in the logs but all I see is the standard "link is down" stuff in system.log.

Could I have a bad ethernet jack? I'm getting a USB adapter for ethernet tomorrow, maybe that will get me running - but I want a working ethernet jack. Does anyone have any suggestions?

Specifically: iMac G5, OSX 10.4.8
iMac --> Netgear GS105 switch --> Smoothwall box --> Internet

restart. then try.

if not take it into apple and let them have a look or ring them up