I have been having the same problem that everyone else has been posting about for about 2 weeks now. I have read almost every post with have done every solution that has been posted by to no avail. For that reason I am posting with the necessary information.
Modem brand & model number: RCA DCM425 (provided by comcast)
Router brand & model number: Cisco Linksys E2500
If Wireless, are you using a white or black external adapter, or an internal (Xbox 360S) adapter? White external adapter
Things you have tried: Power Cycling xbox, all network equipment, redownloading profile, reset network settings.
Is UPnP Enabled? Yes/no/Unknown YES
Do you have a NAT error? If so, what is your NAT? NO ERROR
Any Error Codes you encounter: NONE
Who is your ISP(Internet Service Provider)? COMCAST
After you run "Test Xbox LIVE connection" press “Y” for "More Info”. In the pop-up that appears, what are the following values, if shown? (If a value below does not appear for you, just leave it blank.)
Q: 8015- 100D
D: 18.104.22.168, 22.214.171.124