How can I fix my xbox live connection?

Ok, so my NAT type has always been moderate, or if my step brother is on before me, strict. And it has never lagged or anything else like it has been. Now i'll end up having 1 or 2 bar connection, or even 3 and still lag. I've gone all over on xbox.com and had done everything. How can I make it where my NAT is open (if you know how) and most importantly how can I get it so Call of Duty will STOP lagging for good!? Thank you so much.