CM1000 Xfinity issues

You didn't give nearly enough information for people here to help you troubleshoot. But for what it's worth? I ran into your post while looking for discussions about Netgear CM1000 and CM1150v modems having problems getting the speed Comcast customers are paying for.

I purchased a CM1000 initially and had problems where it was only getting about 400mbits/sec top speeds despite having the gigabit tier of service from Comcast/XFinity.  I switched to the CM1150v since I also have Comcast voice service and realized that modem was essentially a CM1000 with phone jacks on it too, for their voice service.

Same problem though....  Modem can't get over about half the speed I'm paying for, even with a laptop plugged right in the back of it over wired Ethernet that defiinitely can handle 1 gigabit speed.

I still have the XB6 Comcast Xfi gateway they provided when I signed up for gigabit service, and when I set it up instead of the Netgear, I get the speeds I'm supposed to get. A Comcast tech came out and confirmed my line conditions are "excellent", as well -- and couldn't explain why the Netgear modem was so much slower.

I see several other message threads here with the same basic problem, although in at least 1 or 2 situations, it looks like at least PART of the problem was poor signal quality from Comcast.  Still, I've done all the troubleshooting steps and know my circuit works reliably at gigabit speed when using the XB6.  Comcast insists my Netgear modem was properly provisioned, and the tech even called in to verify that with a level 3 person.  I'm about to give up and return this modem, and just resign myself to paying Comcast the stupid monthly modem rental fee -- at least until another product comes out that works better with their system.