Vibration Effect Question

Instead of having the image appear and start vibrating in a random position, how do you have it set up so it remains within a 10 pixel area of where placed on the FLA file, and vibrates there alone?

The image starts is at: (x:71.5,y:-0.5)
and the sizes, which i also want to remain the same, are: (W:91.0,H:111.1)

The code I used is the same from the tutorial… which is:

onClipEvent (load) {
//movie width/height
height = 200;
width = 300;
//------------------------//
//makes everything random //
//------------------------//
this._x = Math.round(Math.random()*width);
this._y = Math.round(Math.random()*height);
var temp = this._alpha=Math.random()*100;
this._xscale = this._yscale=temp;
//setting initiaion position
cx = this._x;
cy = this._y;
}
onClipEvent (enterFrame) {
//causes the object to be offset
this._x = cx+(1+Math.random()*5);
this._y = cy+(1+Math.random()*5);
}