It would be good to see a lspci -n of that machine.

I suspect we'd see this

00:0f.0 1106:0591 ....

The vt6420 seems to have two pci-ids. This was fixed sometime during 2.6.18rc.. ,

It's going on my todo list to a build a Knoppix with the 2.6.18 (Avast!, A bilge rat) release kernel -
as I use motherbaord with this chipset quite a bit.