Dear Hendrik, Dag and Wealid, thank you for the replies; to be more specific, here is some more info: its a DELL Optiplex 755, with a 2 Gb/s onboard card. I did not write this because the same happens with an additional, elder 3Com card and Fedora 9, with a 2.6.25 kernel. Monitor and SATA disk work fine.

The computer is on a local switch together with an older computer, which in turn communicates well at 25 MB/s with all the rest of the world. At Dell's they propose a hardware support, however not for that model. Nevertheless they support officially CentOS. That's why I asked whether anyone had encoutered the same problem already, and could perhaps provide a solution.

Yours, Peter