I was couple of weeks ago on a LAN-Party, 2 guys had the same motherboard from ASUS. Don't remember the exact type but both where for the AMD processor, and had onboard LAN.
The problem existed of having exactly the SAME MAC adresses at both motherboards (00-01-02-03-04-05), those 2 got the same IP adress from the DHCP Server, and had problems with the network. After this was solved (with a new 3Com NIC in one of the two), they finally had no problems with the network anymore.
We've tried to put in a manual MAC adress, but that gave problems with the driver. No access at all to the network after that.
We could not find any clue to why this problem was, the most plausable one was that the NIC chip onboard of a serie of motherboards not was customized with a unique MAC address. As the Motherboards where not mine, I spend no time further to investigate this. Your post here reminded me of that problem back there.
The same MAC adresses give problems with identifying who's who. The DHCP gives IP adresses out on base of the MAC adres. Every time a MAC adres requests a IP, it looks it up in it's list, and gives back the same IP.
I would say, check your MAC adress, if it has a "strange" number like the one I mentioned above, you may have this problem b'cause someone on the same network (neightbourhood on the cable) has the same motherboard, and maybe therefore the same MAC adress. This path looks not promising to me, as with a cable modem you should have a NAT/private network. In which your computer would have nothing to do with the other computers on the (inter)network.