onClipEvent (load) {

    var Xtarget = random(301); 

}


onClipEvent (enterFrame) {

    // i can also separate the motion 

    // and the resetting of the variable

    if(_x > Xtarget){

        _x = _x - 10;

    } else {

        _x = _x + 10;

    } 

    // if the instance is within 15 pixels

    // of the target value, set a new target

    if(Math.abs(_x - Xtarget) < 15){

        Xtarget = random(301);



    }

    // Math.abs(somenumber), a method of the
    // Math Object, returns 
    // the absolute value of "somenumber"

}