Roll a cube help

I have written a script to rotate a cube on keypress which looks like this:

 function move (v : Vector3) {
     isRotating = true;
     start = transform.rotation;
     v = v.normalized;
     n = start * Vector3.Cross(Vector3.up,v);
     c = Mathf.Cos(Mathf.PI/4);
     s = Mathf.Sin(Mathf.PI/4); 
     rotate = Quaternion(n.x*s,n.y*s,n.z*s,c);
     i = 0.0;
        rate = 1.0/rotationRate;
        while (i < 1.0){
       i += Time.deltaTime * rate;
       transform.rotation = Quaternion.Lerp(start, start*rotate, Mathf.SmoothStep(0.0, 1.0, i));
       yield; 
        }
        transform.rotation = Quaternion.identity; //dont want to use!!!
     isRotating = false;
 }

My problem is that I have to resort to a reset of the rotation after call or the angles lose precision and also I start rotating the cube incorrectly. Does anyone have any ideas on how I can fix the code so that it doesn't need the reset.