Random movement in defined area

I want to have about 15 verticle lines moving randomly along the X axis

Now I have modified the Kirupa random movement tutorial to this code:

onClipEvent (load){
//data you may want to change 
width =  300;
speed =  Math.round(Math.random()*4)+1;
 //initial positions 
x =  this._x=Math.random()*width;
x_new =  Math.random()*width;
onClipEvent (enterFrame)  { 
//x movement 
if (x_new>this._x)  { 
sign_x =  1;
 } else  { 
sign_x =  -1;
dx =  Math.abs(x_new-this._x);
 if ((dx>speed)  || (dx<-speed))  { 
this._x  += sign_x*speed;
 } else  { 
x_new =  Math.random()*width;

But I noticed the starting point of the verticle line is always in the same spot and is not confined to where I want it positioned…How would I define an exact stage for it be contained in?

Also it goes everywhere on the stage just not what is defined as the width in the begining of the code,