I've spend about 2 hours with the online tech support from Xbox yesterday what we've tried;
Powerrecycling (switching everything off for few minutes)
Deleting my profile and recovering it
Clearing memory cache
Opening Ports to make sure that the NAT is open
Clearing the network settings back to factory settings
Switching xbox off separately for few minutes
Switching the router off separately for few minutes
Registering a new account with the month trial (I did it personally to see whether it was restricted to my account or Xbox in general) - It was the xbox, because the new account struggled too.
Trying various other games - BF3/COD/AC:R
Cleaning my hard drive of any faulty items (The ones with the exclamation mark)
Troubleshooting my connection about 5 times to make sure I am connected to live.
And that's what I just recall off the top of my head. Fucking ridiculous.
What's worse is people on the official Xbox forums are being assholes and ignoring my problem - even the staff themselves, I think it's because they're clueless as to what to do there anymore, as I listed practically every method of troubleshooting I've attempted.
Disconnect and self destruct, one bullet a time.
