Random moving within stage are v2

Hi, Experts

I try to make a MC to move randomly. How do I make it move only within the stage area ? if it is possible to modify the code      speed = random(5)+8;

Thanks

Fla @ http://www.4shared.com/file/8344914/f5d6fcae/jet_perspective_fly_test.html

Code:

function changeDirection() {
     trace("Here is change");
     // X and Y within the Stage area
     //newX = random(Stage.width-_root.jet_img.ene_jet._width)+_root.jet_img.ene_jet._width*20;
     //trace("New X = "+newX);
     // left and right
     dirX = Math.floor(Math.random()*4);
     trace("X Direction = "+dirX);
     // top and down
     dirY = Math.floor(Math.random()*4);
     trace("Y Direction = "+dirY);
     // Speed
     speed = random(5)+8;
     trace("Speed"+speed);
}
intervalId = setInterval(this, "changeDirection", 300);
mouseListener = new Object();
_root.onEnterFrame = function() {
     // X
     if (dirX == 0) {
          // Down
          _root.jet_img.ene_jet._x = _root.jet_img.ene_jet._x-speed;
          _root.jet_img.ene_jet.gotoAndPlay("left_turn");
          trace("Jet X Position = "+_root.jet_img.ene_jet._x);
     } else if (dirX == 1) {
          // Up
          _root.jet_img.ene_jet._x = _root.jet_img.ene_jet._x+speed;
          _root.jet_img.ene_jet.gotoAndPlay("right_turn");
     }
     // Y                                                          
     if (dirY == 0) {
          // left
          _root.jet_img.ene_jet._y = _root.jet_img.ene_jet._y-speed;
     } else if (dirY == 1) {
          // Right
          _root.jet_img.ene_jet._y = _root.jet_img.ene_jet._y+speed;
     }
};