I upgraded to a DPC3825 some time ago. Since then I have had issues with my Xbox not being able to connect to Live. I haven't used the Xbox for a long time but decided to give it another go to see if I can resolve this issue.
The Xbox is wired into a bridge that is wirelessly connected to the DPC3825. This worked fine with the previous setup which was a wireless router connected to the cable modem. After the switch to the 3825 the Xbox cannot connect, claiming DNS failure. To further eliminate the bridge I tried a another Xbox with a built-in wireless connection that connected directly to the 3825, it had the exact same issues. I also tried a wireless dongle attached to the Xbox to directly connect to the 3825, same issues. So I am ruling out the bridge. I only thing I cannot try is the Xbox hard-wired to the 3825 since it is on another floor and I don't have a display there for the Xbox.
Other devices like Win8 laptops and Win8 phones can connect to Live without issue. So the problem is localized to the Xbox.
I spent some time trying different things, ended up with everything manually configured on the Xbox. Static IP, manual DNS IPs, etc. Still no luck. Tried the public Google DNS IPs (22.214.171.124 and 126.96.36.199), no luck. The Xbox always claims that the DNS servers cannot resolve Live server names. I added Port Forwarding in the 3825 for the Xbox's IP as well. Tried putting the Xbox IP in the DMZ as a last resort. No luck.
After I power cycle the 3825 I sometimes get about 3-5 minutes of connectivity before the Xbox drops, after that it will not connect again. This leads me to believe it is something in the 3825, not sure what though. All other internet activity is without problems. I have a 100Mbs connection and can stream HD movies, get great download speeds and have few connectivity issues across a wide range of devices. Wireless signal is clean, no interference, majority of devices in the house are wireless and generally have no issues. Just the Xbox doesn't connect to Live!
I am out of things to try. Any other suggestions welcome, I do want to get this sorted somehow.