Take a look at the output of 'lspci -v'. I'm wondering if the two NIC cards are using the same interupt. This may be a problem if you have isa cards or don't use acpi.
Here is a FAQ on ethernet with a section on problems with two NIC cards. Maybe it will help.
http://www.faqs.org/docs/ethernet/Ethernet-HOWTO-2.html
Another Idea, (I'm assuming you have Mandrake Linux) is to make a rescue cd with the command
mkrescue --iso. Then burn the cdrom image. There is a diagnostic menu selection that will look at your system and suggest needed kernel boot options. It just may notice something.