哪位大神，帮我看看，为什么我的JS不能滚动啦？

<div class="pre">
</div> 
<div class="next"></div>
<div class="smalltitle">
  <ul>
    <li class="thistitle"></li>
    <li></li>
    <li></li>
    <li></li>
    <li></li>
    <li></li>
  </ul>
</div>
<ul class="oUlplay">
  <li><img src="images/slide1.png" width="646" height="303"/></li>
     <li><img src="images/slide2.png" width="646" height="303"/></li>
     <li><img src="images/slide3.png" width="646" height="303"/></li>
     <li><img src="images/slide4.png" width="646" height="303"/></li>
     <li><img src="images/slide5.png" width="646" height="303"/></li>
     <li><img src="images/slide1.png" width="646" height="303"/></li>
</ul>

function getStyle(obj,name)

{
if(obj.currentStyle)
{
return obj.currentStyle[name]
}
else
{
return getComputedStyle(obj,false)[name]
}
}

function getByClass(oParent,nClass)
{
var eLe = oParent.getElementsByTagName('*');
var aRrent = [];
for(var i=0; i<eLe.length; i++)
{
if(eLe[i].className == nClass)
{
aRrent.push(eLe[i]);
}
}
return aRrent;
}

function startMove(obj,att,add)
{
clearInterval(obj.timer)
obj.timer = setInterval(function(){
var cutt = 0 ;
if(att=='opacity')
{
cutt = Math.round(parseFloat(getStyle(obj,att)));
}
else
{
cutt = Math.round(parseInt(getStyle(obj,att)));
}
var speed = (add-cutt)/4;
speed = speed>0?Math.ceil(speed):Math.floor(speed);
if(cutt==add)
{
clearInterval(obj.timer)
}
else
{
if(att=='opacity')
{
obj.style.opacity = (cutt+speed)/100;
obj.style.filter = 'alpha(opacity:'+(cutt+speed)+')';
}
else
{
obj.style[att] = cutt+speed+'px';
}
}

},30)

}

window.onload = function()
{
var oDiv = document.getElementById('playBox');
var oPre = getByClass(oDiv,'pre')[0];
var oNext = getByClass(oDiv,'next')[0];
var oUlBig = getByClass(oDiv,'oUlplay')[0];
var aBigLi = oUlBig.getElementsByTagName('li');
var oDivSmall = getByClass(oDiv,'smalltitle')[0]
var aLiSmall = oDivSmall.getElementsByTagName('li');

  function tab()
  {
     for(var i=0; i<aLiSmall.length; i++)
     {
        aLiSmall[i].className = '';
     }
     aLiSmall[now].className = 'thistitle'
     startMove(oUlBig,'left',-(now*aBigLi[0].offsetWidth))
  }
  var now = 0;
  for(var i=0; i<aLiSmall.length; i++)
  {
      aLiSmall[i].index = i;
      aLiSmall[i].onclick = function()
      {
          now = this.index;
          tab();
      }
 }
  oPre.onclick = function()
  {
      now--
      if(now ==-1)
      {
          now = aBigLi.length;
      }
       tab();
  }
   oNext.onclick = function()
  {
       now++
      if(now ==aBigLi.length)
      {
          now = 0;
      }
      tab();
  }
  var timer = setInterval(oNext.onclick,3000) //滚动间隔时间设置
  oDiv.onmouseover = function()
  {
      clearInterval(timer)
  }
   oDiv.onmouseout = function()
  {
      timer = setInterval(oNext.onclick,3000) //滚动间隔时间设置
  }

}