You state you have a DHCP MAC address, how do you know that? If you are running a DHCP server on your network this may be the culprit. Are you sure that you don’t have a rouge DHCP server on the network? Also there are sniffer tools/programs available. Based on the information you have provided this is the best information I can provide. Any network administrator can find a rouge router or DHCP server on their network. One last question is this a WIFI setup?