Welcome. Since my
Xbox I have a tremendous trouble with the
matchmaking (usually only in the games
Call of Duty).
The point here is that NAT is open but like to join a server takes a good few, and even a few minutes. With BF3 or MoH do not have a problem. The PC also does not have any problem with the matchmaking in MW3
This video should explain my situation to you (this is exactly the same) (for the impatient of 0:50 sec.)
A picture of the test here connection to my Xbox:
Xbox is connected by cable to the router.
My router is a TP-Link W8901G (updated to the latest FW, UPNP enabled, NAT enabled, Firewall off)
I hope for quick help.
ps. sorry for my bad english.