As far as I know, it is an issue with the 802.11n networks - the 360 can't handle it - there's good ol' Microsoft for you... Anyway, I have worked around this issue quite nicely with an Airport Express that I had. Just bridge the Express to the n router (in my case it was an AEBS), creating a secondary b/g network which works seamlessly with my 360. You'll be back to playing Call of Duty 4 again in no time!