I've got a HALO 4 XBOX 360 which I've had since launch (Oct 2012) and have been plagued with WiFi issues for the last 6 months.
I've bought repeaters/access points and recently a range extender thinking somehow a black spot has developed. Maybe a local network was causing interference...
The 5GHz band is perfectly fine, it's just 2.4 GHz that suffers with regards to speed.
It's recently gotten so bad that a 75Mb/s connection was giving be <10Mb/s right next to the router! And <4Mb/s a few meters away in my room.
So after a process of elimination I came to discover that unplugging the 360 from the wall meant the WiFi jumped up to 45Mb/s in my room.
I have tried googling the problem but don't get any hits.
I might have to call Microsoft and get them to send me a new PSU.