I can't start a lobby with my friends or play multiplayer at all since the last update on both IW and MWR. Both games my NAT Type will be either Strict or Moderate, but according to my network settings on my Xbox, it's open. These are the only two games that don't work and have this problem. Really glad I payed $100 for this crap.
You can find some NAT solutions here.
Hopefully this will help. I used to have similar issues but with port forwarding I was able to get my CoD NAT to stay as open.