My router: Linksys WRT160N Wireless-N router
As far as this problem, I have already covered:
-NAT is open
-Channel 4 to avoid wireless interference from other electronics
-MTU settings are set properly
-Router is in elevated position on 1st floor, Xbox 360 is near ground-level on 2nd floor
Xbox live, in most of it's entirety, works fine on my new wireless set up. There were NO problems before when I ran a wired connection from the router, but have since been forced to utilize a Wireless-N external adapter as a wired connection is currently not possible.
Connecting to Xbox live is not an issue; except for the occasional disconnect. I do not experience lag.
The problem is that within games, they always have problems contacting the game's server. For instance, in Halo Reach; I will be booted out of matchmaking due to networking issues; I'll get messages saying the server is temporarily unavailable, and at the end of games my credits will be calculated like I'm in offline mode. This happens about 33.3% of the time.
In Modern Warfare 3, I will occasionally be kicked out of matchmaking, and even mid-game, saying the server is currently unavailable.
In Soul Calibur 5 it never connects to the server, and often fails to join game lobbies, and when I do, it can't connect to the host.
My computer is located a few feet away from my Xbox 360, and also uses a wireless connection to the router. It has not experienced any connection issues. The Xbox 360 is the only one doing so.