This is going to sound stupid, but check your NAT. I say that not because I doubt that you have it set to open, but because I'm wondering if MW3 says your NAT is set to Closed or Moderate. I have all PS3 and 360 ports open on my NAT. All my games say my NAT is open, but for some reason Call of Duty games constantly screw me up. Black Ops says I have an open NAT, yet for some reason I cannot join the party of my friend (even though I can in every other game we own). Then, the other day when I was playing online and noticing that it was taking forever to even find me a match, I saw that it said I had a Moderate NAT. I checked, and everything's still open. So I'm wondering if you're having the same problem I am? And if you are, I feel your pain. Let me know if you come across a fix, because I'm betting this is something on CoD's end.