Changing NAT Type - Tried Everything

I've tried to change my NAT type from 3 to 2 in order to connect to MW2 MP (for the record - all other online games work even with NAT 3. MW2 also worked with NAT 3 until a couple of weeks back). I've set up a DMZ, and my PS3 now has a static IP address. I've opened all the ports mentioned in the PS3 manual and the MW2 port as well. When I run an internet connection test, I still get NAT 3. I'm really all out of ideas.

Does anyone have ideas on what I can do ?

EDIT: I still can't connect to MW2 MP, but all other games are fine.

Your modem may have it's own firewall, DSL modems tend to use them. Search the web to find out how you can verify if this is the case for your broadband service and modem model and disable the firewall if needed.