Xbox 360- I've tried everything and still have a strict NAT?

I use a Linksys WRT160N router, version 3.0.03. I have opened ports 1-4000, both udp and tcp. I have set up a dnz from any IP direct to my Xbox's IP. Upnp is enabled. Firewall is disabled. Is there anything else I can possibly try to get my NAT open. If I have to buy a better router, fine. I don't care anymore.