Off beat post this.
My sons Xbox started misbehaving the other day, and refused to run the Call of Duty games connected to XBox Live. The games would start up Ok, but then disconnect from XBox Live – not good. Googled a lot but not much out there (a lot about it disconnecting during games but not on start up.)
General advice was to clear the system cache, or wait at the dashboard for a while, open ports on the router, reset the console, recover gamertag, but these didn’t work. Other games like Team Fortress would play OK, though.
Finally I tracked down the solution, which was to set a FIXED IP Address for the XBox on the router. It had been working fine previously on DHCP, but suddenly stopped working. The fixed IP address resolved all the issues on all the games.
I am an utter cult