My NAT changes at weird times ( from moderate to strict and vice versa) I think this is because of the internet having slower and faster pings. Not sure tho
reason you get strict is because there is way to many equipment connected to your router that use wifi example: laptop,pc,phones,tv and so on
most of them have an ip range between 192.168.1.0 and 40 if you go to your ps3's internet settings go to custom and set a static ip to your ps3 (i use 192.168.1.160) then
you would have more often a better nat because there will never be something that conntects with 160 and above to your router anyway in order to always get open
use dmz mode and set the dmz mode to your ps3's new static ip