whats the tyre code you've got on. Could be the cupping issue.
Easiest way to tell is to look at the outer tread is the groves runing perpendicular to the direction of the wheel go all the way to a groove running with the direction of the tyre, then you have a old style tyre and if you car is under warranty, Nissan should replace them. It's a well know issues and make sure the front toe is adjusted to new spec (2mm toe-in). Nissan also should carry this out to.
If the groove doesn't run all they way the groove running along the direction of the tyre and stops in a block of rubber then you have the new style. You won't have a cupping issue and will be something else that is causing the problem
more info here
http://www.bonell.plus.com/350z/tire%20 ... lletin.pdf