Just a heads up to anyone who has been having connection issues with MW3 MP, I had a 'moderate' NAT, it would struggle to find games with a low ping, often would connect to a game, load map then time out.
SO anyway after a few hours or changing things I simply disabled upnp on my router (had previously tried manual port forwarding amongst other things). Disabling it left my NAT status as 'open' and now connects to lower ping matches and the time outs are now pretty much non-existant. Strange as I was under the impression the whole point of UPNP was to auto-configure thing such as NAT?
Anyhow TL;DR If your having connection problems or timeouts try disabling UPNP.