Ok I found the web page that helped me back then.
http://www.rasmithuk.org.uk/?page_id=81%3Cbr%3E
My router is a Netgear WGR614 v6 . In its original firmware it used something called a symmetrical shaped NAT which you can read about in Russ' blog. So I HAD to have a strict NAT on xbox live even though it was telling me it was open but I was always getting blocked from joining any game which had my friend, that knew his NAT was moderate, already in it. I had tried all the static adresss, DMZ, port forwarding tricks which only caused the dashboard test to say I had a Strict NAT. I had to use PnP to get the dashboard to falsely claim an open nat.
Anyway I later found that the newest version of firmware for my router enabled it to be switched to a Cone shaped NAT. So I downloaded it wrote down all my IP and security settings and installed it from the routers upgrade section, it was very easy. Router then rebooted, and I re-did all my security settings and addresses. I then found the new cone shaped NAT box and ticked it. Since then ive never failed to join a game, I still only use PnP so I don't need to open ports or mess with the DMZ.