First Z woes of the year...

Use what Nissan suggests. Ever motor responds differently to different plugs.
I bet every guy on here that is saying run coppers can also change their plugs in about 5min. For people like us, it is not reasonable to change plugs with every oil change, and most people who I know that run coppers have to change them about that often. But to me that iridium does not seem to be the likely problem as much as it could be gap or heat range.