The router should definately NOT have the MAC adress of the cable modem cloned into it. It should either be left at default as Monkeybreath said or have the MAC address of a connected system network card cloned in.
As it is getting late and I am going to bed, if you are still about have a read of Robin Walkers guide at
http://homepage.ntlworld.com/robin.d.h.walker/cmtips/
If that doesn't help you then someone will be about later or in the morning