Let-vægts galleryshow fejl?

Hej jeg er i gang med at lave en flot og let udgave af en slags 'lightbox'.

Men dette er hvad der nogengange sker:
http://img329.imageshack.us/img329/4937/unavngivetww8.png

det sjove/mærkelige er at nogengange virker det


Javascript:

function ShowImage(url,comment)
{
  document.getElementById('imgholder').style.display="block";
  document.getElementById('imgholder').innerHTML="<img src='"+url+"' alt='Billede' /><br /><br /><hr class='blackstripe' /><br /><p class='blacktxt'>" + comment + "</p><br /><hr class='blackstripe' />";
  document.getElementById('temp').innerHTML="<img src='"+url+"' alt='Billede' id='tempimg' />";
  var width = document.getElementById('tempimg').width;
  var height = document.getElementById('tempimg').height;
#imgholder
{
  position: absolute;
  z-index: 3;
  margin:0px auto;
  padding: 20px;
  padding-bottom: 10px;
  background-color: #FFFFFF;
  display: none;
  text-align: center;
}  document.getElementById('imgholder').style.width=width;
  document.getElementById('imgholder').style.height=height;
  document.getElementById('black').style.display="block";
  heightofscreen = window.screen.height;
  widthofscreen = window.screen.width;
  if(height >= 780)
  {
   blackH = '110%';
   document.write
   document.getElementById('black').style.height=blackH;
  }
  document.bgColor = '#008000';
  
  hPos = (heightofscreen/2)-(height/2);
  wPos = (widthofscreen/2)-(width/2);
  hPos = Math.round(hPos);
  wPos = Math.round(wPos);
  hPos -= 75;
  hPos += "px";
  wPos += "px";
  document.getElementById('imgholder').style.top=hPos;
  document.getElementById('imgholder').style.left=wPos;

}

CSS:

#imgholder
{
  position: absolute;
  z-index: 3;
  margin:0px auto;
  padding: 20px;
  padding-bottom: 10px;
  background-color: #FFFFFF;
  display: none;
  text-align: center;
}

body
{
  background-color: #2970AD;
  background-image: url('http://www.skatetown.dk/SkateDeveloper/images/bg.png');
  background-repeat: repeat-x;

  font-family: Verdana, Tahoma, arial;
  font-size: 11px;
  color: #FFFFFF;
  width: 100%;
  height: 100%;
  padding: 0px;
  margin: 0px;
}