If you really wanted to figure out if it's something coming through your cable modem... Two tests:
One: (really easy)
Unplug your cable modem from the WAN port on your DIR-655 and see if the MAC spoofing continues. Should it stop, it's coming from the WAN. If it continues, then you're likely infected with something.
Two: (relatively advanced)
Download an application called "Wireshark" to a computer that you don't mind having unprotected access to the internet (read: have to potentially erase/restore), read the manual/help for a moment, then..
If you have a spare ethernet hub 10/100 sitting around (and it has to be a hub, not a switch), you can put the hub between your DIR-655 and cable modem. Also attach said computer. Fire up Wireshark, and start capturing packets. Record for a few minutes, unplug the ethernet cable, stop capturing.
Look through the log (filter) for the MAC address sequences and see if you can find an IP address as a source for the MAC spoofs. If there is a single IP address that keeps spitting out the MAC addresses, then you have found your attacker. File a complaint with your ISP.
You may also find other interesting things, maybe not. It's a crapshoot, though you'll get some interesting data, I'm sure.