First I would like to thank you all for posting your issues with this device.
With the fixes that I have read on here some might work for a short time, but your issue will most likely come back.
This device, the Arris TG862, is given out by many companies including CableOne, InSight, Shaw, and Timewarner. They all have had this same issue when it came out. With those companies the have since fixed the issue by using the Arris firmware that has been updated. The Comcast/Xfinity firmware on the other hand has issues with all security types connecting to the Xbox 360s wirelessly. It also has many other issues with the firmware that will cause poor connection to online games once you do get connected.
Until Comcast updates the firmware to a version that will work the best solution for you folks is to exchange that Arris for a different device from Comcast. They do give out other devices such as the Motorola or SMC gateways. Or you can purchase your own modem and router and register it with Comcast. If you call Comcast about this will suggest to go to your nearest service center to swap it out. You can go to this web site to find the one near you. www.comcast.com/.../storelocator.html
Also those DNS issues that StuntmanMike04 is talking about is posted on Comcast's website as well. mydeviceinfo.comcast.net/device.php
Hopes this helps you guys.