XML : XML sometimes returns false?

I have this code to retrieve XML from a file:

  <?php  $lop = curl_init("http://www.stardoll.com/en/xml/getMeDollItems.php?external=1&id=11");  curl_setopt($lop, CURLOPT_RETURNTRANSFER, true);  curl_setopt($lop, CURLOPT_HTTPHEADER, array("Cookie: admin=1"));  $output = curl_exec ($lop);  $xml=simplexml_load_string(trim($output));  if($xml===FALSE){    // didn't work    echo "Did not work!";  } else {    // it worked    echo "Worked";    echo $xml->asXML();  }  ?>    

Sometimes this echoes "Did not work!", but when I reload the page it works (sometimes I have to reload twice). Other times it works normally.

Can someone help me figure out why its doing this and how prevent it from doing it again?

Thanks :)