Did Comcast do any testing when they were trying to configure the Modem?
Have seen a couple times where other people have had that same problem with the 6120 and 6121 coming from the DOCSIS 1.1 modems. Turns out they had a lot of noise ingress and other line problems.
Also see that some modems bought on ebay had been stolen and never can be provisioned.
Go to www.dslreports.com/forum/comcast and post your problem there. There are some Comcast techs and engineers plus a bunch of very knowledgeable people that deal with problems like yours.
I have a Windows 7 computer with Comcast HSI. It's a very fast computer, but the modem was really outdated. So was the router. So, we got a brand new Cisco N+ Router and a Motorola SURFboard SB6121 modem with DOCSIS 3.0 and IPv6. However, when plugged in, the new modem will not consistenly connect to the internet. We called Comcast and they tried to send an activation code to the modem with it connected to the computer directly, but it was intermittently going online, then offline, then back online, then offline. We exchanged the modem for an identical one, but it's doing the same thing, so the modem can't be at fault here. Comcast tried to tell us that something was wrong with our coaxial cable, but our old modem - a Motorola SURFboard 4100 running DOCSIS 1.1 - was working just fine with the same cable. Another technological "improvement" has failed me - what do you think the problem could be?