Don't know about pump, but dhclient works perfectly fine with NTL. It's a bit dodgy that you're getting two totally different MAC addresses though. The only MAC address Linux should see is the MAC address of the card. If Knoppix works, I would say the MAC under Knoppix is right. You may want to check the NIC card driver module under Mandriva. If it's sending the wrong MAC address out when your dhcp client is making a request, the dhcp client is never going to get an answer sent back to it as it needs the MAC address to be correct for this.
Consider trying Ubuntu instead of Mandriva
