URLLoader gets stuck when polling

private function check_server() {
 var url:String = "http://coworkthailand.com/say/snd/index.php?"+Math.random();
 if (loader != null) {
  trace("was already checking "+loader.bytesLoaded+" / "+loader.bytesTotal);
  return;
 }
 loader =  new URLLoader();
 loader.dataFormat = URLLoaderDataFormat.TEXT;
 loader.addEventListener(Event.COMPLETE, completeHandler);
 loader.addEventListener(flash.events.IOErrorEvent.IO_ERROR,
  function(e:Event) { loader = null; trace("fail"); })
 loader.addEventListener(flash.events.SecurityErrorEvent.SECURITY_ERROR,
  function(e:Event) { loader = null; trace("security error"); })
 loader.addEventListener(flash.events.HTTPStatusEvent.HTTP_STATUS,
  function(e:flash.events.HTTPStatusEvent) { trace("status "+e.status); });
 try {
  loader.load(new URLRequest(url));
 } catch (error:Error) {
  trace("Unable to load requested document.");
 }
}

<?xml version="1.0" encoding="utf-8"?>
<mx:Application xmlns:mx="http://www.adobe.com/2006/mxml" layout="absolute" initialize="app_init()">
    <mx:Script>
    	<![CDATA[

    		private var timer:Timer;
    		private var loader:URLLoader;
    		private var isOpen:Boolean;

    		private function app_init():void
    		{
    			timer = new Timer(5000)
    			timer.addEventListener(TimerEvent.TIMER, timer_tick, false, 0, true)

    			loader = new URLLoader();
    			loader.addEventListener(Event.OPEN, loader_open);
    			loader.addEventListener(Event.COMPLETE, loader_complete);

    			// Start the timer
    			timer.start();
    		}

    		private function timer_tick(event:TimerEvent):void
    		{
    			// Check if the loader's busy before calling load, and/or close()
    			if (!isOpen)
    			{
    				// loader.close();
    				loader.load(new URLRequest("http://mydomain.com/myfile.txt"));
    			}
    		}

    		private function loader_open(event:Event):void
    		{
    			// Mark as open
    			isOpen = true;
    		}

    		private function loader_complete(event:Event):void
    		{
    			// Do work

    			// Mark as closed
    			isOpen = false;
    		}

    	]]>
    </mx:Script>
</mx:Application>