Moderate NAT will definitely do that to you. MW2's PS3 servers are pretty shit all around so it helps to have the best possible connection. What you'll want to do is figure out (i.e. Google) what port your PS3 connects through on your router and open that port up so you'll get an open NAT. Usually you can access your router externally by typing 192.168.1.1 into any web browser. From there you can access your port forwarding, enter the port number, and open that port.
Moderate NAT gave me a million headaches on Xbox Live so I'm well versed in its evils >_>.
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