Ever since I phoned virgin media up about slow speeds during the evenings, they said they would "do something to improve connection on Xbox." However, the next day my nat type has changed to unavailable and I cannot join parties to speak to friends. When I gets my connection it says I cannot get a teredo IP address. I have tried enabling dmz, port forwarding, simple resets of the router and hard resets of the Xbox but nothing seems to fix it permanently. Every time I turn my Xbox off and on after getting an open nat it will change back to unavailable.
I'm having the exact same issue. Tried all the above as well and everything else. The only thing that works is by enabling modem mode which means you basically lose all wireless functionality. Seems to something Virgin or the router is doing.
I am having exactly the same issue and it's only occurred in the last couple of days. I too tried all the troubleshooting steps and have also spoken quite a lot to the xbox support team. They're pretty confident it's a router issue.