I very recently bought the same modem. I have the same issue. I have made numerous contacts with Comcast and a contact with Netgear. The issue still persists. Comcast blamed Netgear because everything on their end looks good. Netgear said Comcast has to set the power frequency to the right levels for each channel and ALL channels must be locked. Comcast said they are not allowed to and do not have the tools to make said changes to this router because it is not their equipment. In my case I have 31 of the 32 downstream channels locked and 4 of 8 upstream channels locked. According to netgear the power levels for each downstream should be between -7 and +7dbmv which I think at the time of the call they were on the high side. The power levels fo the upstream should be between +38 to +48dbmv. Which mine are slightly higher than 48dbmv. Comcast wanted me to call Netgear again. I told them I was not going to bounce back and forth between the 2 companies. I plan to send back the modem. The issue is do I get a new Netgear or go with the only other option available which is an Arris which as less capability? I am not sure what to do here, but I am considering cancelling service and finding another provider.
I'm surprised you haven't had a tech scheduled out there. If you're willing to put some work in, check your connections. Start with a single run from the outside box straight into your modem. Check the grounds on the connection outside as well. Run a speedtest, check the power levels on the modem. Most likely probably a bad splitter or line between your house and street needing to be replaced. Splitters should be 5-1ghz(recommend antronix). Any unused connections on splitters should be terminated as well.
I use to be on the 500mbps using a NetGear CM1000. Until they buried a new line outside, I was getting very bad intermittent outages on a daily basis. Now I upgraded to the triple play with 1gbps plan, was seeing 600-650 mbps with a XB6-T(CGM4140COM). For two weeks calling back in fourth, doing the same diagnostic checks. I got fed up and went to best buy and purchased a CM1150v. I can tell you this modem is worth the money, i'm now getting ~960 downstream and 43 upstream steadily. During my whole adventure, I ran a new line from the box outside the house to my new network closet, as well as replacing the splitter outside(cable box in living room). Connected the other end of the splitter to a Antronix MRA4-8 amplifer, terminated and grounded properly since it is next to my breaker box.
After attempting to setup a second modem of the same model number it was determined a tech was needed. They did send a tech and he checked things out. Come to find out I had a "bad" line that was installed by an electrician during construction of the home. So the tech ran a new line and got the modem to work. However, 2 days later the issue is still unresolved. My speed after a little while drops to around 25 to 30 mbps when I am paying for 600 mbps. I have no idea what else to do. I have done everything I can think of to get this resolved. I am not sure what to do next. So I haven't yet called Comcast again. I know that after 2 weeks of trying to resolve this issue, I will probably get the same responses that I've already tried. I do not have the time to keep spending hours on the phone with someone who can't seem to solve the problem. I am very frustrated and a bit exhausted.
Amps aren't always the blanket fix for everyone, sorry. The OP hasn't returned. Dead thread now being closed.