wired DSL connection only lasts 10 seconds

My dads imac has recently started having a problem with the internet. So on Fathers day I worked on it for the entire afternoon, 4 hours I had an ATT tech working along side. My Dad CoreDuo 20" only stays connected for 5-10 seconds and then stops. If I unplug the network cable and reinsert it, I get another 5-10 seconds of the internet. This is via ATT uverse, and a wifi signal is present and allows seemless internet connections, as checked with an ipad. it is repeatable...just unplug and replug the cat5 cable and I always get a few seconds of internet traffic, usually just enough for 1 or 2 pages to load.

Thanks in advance for your suggestions.

Not much information to go on... but try running the Apple Hardware Test on your father's Mac. You'll need the original install disk set in order to do that. Apple Hardware Test

What happens if you disconnect the cable and connect wirelessly?

Are you connected directly to the modem or via a router?
If you have modem and router, what happens if you connect the modem directly to the Mac?

I will give him a call and have him locate the Hardware test disc, I'm guessing his Imac must be 6 years old by now, so I suppose something may have failed.. The imac is plugged directly into the UVERSE modem, which also provides WiFi along with 4 cat5 connections. His Imac doesn't have any wifi capabilities....although that should be cheap and easy. I was able to connect wirelessly with another device and it was fast and stable, and we tried 4 different modems, each had the same issue.

If it's this one it has wifi built-in. Though it will be labelled Airport not wi-fi.

I'm going to take a dig into my memory and say tentatively that all Intel Macs do.