[Vicious]

Since you're asking so nicely... not

var timerMove = null;
 
function moveDown(){
    var items = document.getElementById("my_sliding_div");
    
    if (timerMove == null){
        timerMove = window.setInterval(function(){
            doMoveVertical(30,1,0,items);
        },5);
    }
}
 
function moveUp(){
    var items = document.getElementById("my_sliding_div");
    
    if (timerMove == null){
        timerMove = window.setInterval(function(){
            doMoveVertical(30,-1,-480,items);
        },5);
    }
}
 
function doMoveVertical(step,sign,limit,items){
    var curTop = items.style.top.replace("px","")*1;
    if ((sign > 0 && (curTop < limit)) || (sign < 0 && (Math.abs(curTop) < Math.abs(limit)))){
        items.style.top = (curTop + (sign*step)) + "px";
    } else {
        // stop moving:
        window.clearInterval(timerMove);
        timerMove = null;
    }
} 

Don't expect support; I'm only giving this script because I had it right in front of me. Next time do us a favor, and either post in the script requests section, or show us you did an effort.