I am not a general expert on Linux networking, but I know enough about nic drivers to strongly doubt that the driver or card are to blame for this.
I suggest a deeper exploration of the options to the iptables command, because "iptables -L" barely scratches the surface of what it can tell you. It is actually my go-to tool for diagnosing any network problem regarding the forwarding of packets in Linux. I would give more details, but I don't know the options off the top of my head and I am typing this on my phone. Good luck! /* PLUG: http://plug.org, #utah on irc.freenode.net Unsubscribe: http://plug.org/mailman/options/plug Don't fear the penguin. */
