DMZ Instructions:
1) Login to your Router.
2) Look for "WAN Settings/Configuration".
3) Find where it says "Default DMZ Server".
4) Get the IP Address of the PS3 (it will be under Connection Status on the PS3).
5) Put the PS3's IP Address into the "Default DMZ Server".
6) The Router should reboot with the changes applied, if not just save the changes and reboot the Router manually.
7) The NAT should become less strict now as you're enabling the PS3 to have free-flow of incoming/outgoing connections.
It fixed my brothers Black Ops problems when connecting with friends, hopefully it'll work for you.