URLLoader problem

Hello, I just started learning flex and I'm working on an issue that has me stumped.  I want to make a request to an application at a url address and I'm expecting an XML response back. 

I was able to get the expected response back using URLRequest, but only if I use navigateToURL.  When I use URLLoader I get a bad response.  Here's my code.

------------

var params:URLVariables = new URLVariables("username=user@email.com");

var request:URLRequest = new URLRequest();

request.url = "http://mydomain.com:8180/functiondir/function"

request.method = URLRequestMethod.POST;

request.data = params;

var urlLoad:URLLoader = new URLLoader();

urlLoad.addEventListener(Event.COMPLETE, ResultHandler);

urlLoad.addEventListener(IOErrorEvent.IO_ERROR, FaultHandler);

urlLoad.addEventListener(HTTPStatusEvent.HTTP_STATUS, HTTPHandler);

urlLoad.addEventListener(SecurityErrorEvent.SECURITY_ERROR, SecHandler);

urlLoad.load(request);

//navigateToURL(request);

---------------

The functions I defined in the event listeners print out diagnostic information.  If I uncomment the line above a browser window will open with the correct XML response in it.  But when I use the urlLoad.load function I get this response in firefox:

----

HTTP Response: '0'

HTTP Done

Error during submit: 'Error #2032'

Error Done

-----

Curiously, if I run the same script in IE8 I get:

----------

HTTP Response: '400'

HTTP Done

Error during submit: 'Error #2032'

Error Done