var timeout = new Array();
var step = 3;

function down(id) {
   var e = document.getElementById(id);
   var y = e.scrollTop;
   if (y + step <= e.scrollHeight - e.offsetHeight) {
       e.scrollTop = y + step;
       timeout[id] = setTimeout("down('" + id + "')", 20);
   }
}

function up(id) {
   var e = document.getElementById(id);
   var y = e.scrollTop;
   if (y - step >= 0 ) {
       e.scrollTop = y - step;
       timeout[id] = setTimeout("up('" + id + "')", 20);
   }
}

function stop(id) {
   clearTimeout(timeout[id]);
}