Transform.Rotate() problem : Avoid Z-axis rotation ?

Hey there ! :)

In the same time I'm working on that car stuff (if your are used to WheelCollider, your help is welcome here !), I'm working on an first person controller. I use the Transform.Rotate(Vector3 v) method using Touch data. It works pretty well globally. However, no matter what I do it keeps rotating around Z-axis which I don't want. I tried to force the eulerAngle to 0 at the end of the Update() but it's not working either.

Any idea ?

Thanks in advance !