Why does my mini not connect properly to the internet?

Since a few weeks my macmini 1.83 Coreduo, with 10.6.8, looses the cabled internet connection.

On the switch I can see it when the lights go off. I then have to disconnect the jack in the switch and connect again. Sometimes this works. At startup the connection never works, I always have to disconnect the cable and connect again. It is not the connection direct into the mac but always the connection into the switch. I also tried the direct connection from the cable modem into the mac, same result!

I changed cables, switches, reset my cable modem, did a clean install, switched users but nothing works.

My printer and macbook pro have no problems at all on the same switch and cables.

What can cause this problem and how can I solve it?