Ajax xmlHttpObject.open() call works in Mozilla but not in IE

Attached is the function that I use and I've not had it fail in any testing.

    function createXmlHttpRequestObject() {
      // create new XMLHttpRequest Object
      var xmlHttp == false;
      
      // will store the reference to the XMLHttpRequest object
      // this should work for all browsers except IE6 and older
      try {
        // try to create XMLHttpRequest object
        xmlHttp = new XMLHttpRequest();
      }
      catch(e) {
        // failed to create XMLHttpRequest object
        // assume IE6 or older
        var XmlHttpVersions = new Array("MSXML2.XMLHTTP.6.0",
                                        "MSXML2.XMLHTTP.5.0",
                                        "MSXML2.XMLHTTP.4.0",
                                        "MSXML2.XMLHTTP.3.0",
                                        "MSXML2.XMLHTTP",
                                        "Microsoft.XMLHTTP");
        // try every prog id until one works
        for (var i=0; i<XmlHttpVersions.length && !xmlHttp; i++) {
          try { 
            // try to create XMLHttpRequest object
            xmlHttp = new ActiveXObject(XmlHttpVersions[i]);
          } 
          catch (e) {}
          // failed to create object
          // do nothing and try the next
        }
      }
      // return the object
      // returns false if we were unable to create any object
      return xmlHttp;
    }

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copy and paste the following:

var xmlHttpObject = getXmlHttpRequestObject();
 
/**
 * Create XMLHttpRequestObject.
 **/
function getXmlHttpRequestObject()
{  
        var xmlHttp=null;
        try
        {    
                alert("// Firefox, Opera 8.0+, Safari");    
                xmlHttp=new XMLHttpRequest();    
        }
        catch (e)
        {       alert("// Internet Explorer");    
                try
                {             
                        xmlHttp=new ActiveXObject("Msxml2.XMLHTTP");      
                }
                catch (e)
                {      
                        try
                        {        
                                xmlHttp=new ActiveXObject("Microsoft.XMLHTTP");
                        }
                        catch (e)
                        {        
                                alert("Your browser does not support AJAX!");        
                                return false;        
                        } // end catch
                }   // end catch
        }  // end catch
        
        alert("xmlHttp:" + xmlHttp);
        
        return xmlHttp;
} // end function
 
/**
 * Called when the AJAX response is returned
 **/ 
function handleResponse() 
{       
        // Check to make sure our response is ready
        if (xmlHttpObject.readyState == 4) 
        {       
                //alert('responseText:' + xmlHttpObject.responseText);
                document.getElementById('speedResultTable').innerHTML=xmlHttpObject.responseText;
                
                //$('#speedResultTable').flexigrid({height:auto});
        } // end if
} // end function
 
/**
 * Get speed data.
 */
function getSpeed()
{
        alert("startTimeToBeSent: " + startTimeToBeSent + 
                        ", endTimeToBeSent: " + endTimeToBeSent + 
                        ", pairIdToBeSent: " + pairIdToBeSent
                );
        document.getElementById('speedResultTable').innerHTML='<tr><td><img src="img/ajax-loader.gif"></td></tr>';
        
        alert("xmlHttpObject:" + xmlHttpObject);
        if(!xmlHttpObject)return false;
	   
        // This line will be called when we receive our AJAX response.
        xmlHttpObject.onreadystatechange = handleResponse;              
 
        xmlHttpObject.open("GET", 
                        'getSpeed.jsp?startTime=' + encodeURIComponent(startTimeToBeSent )+ 
                        '&endTime=' + encodeURIComponent(endTimeToBeSent) + 
                        '&pairing=' + encodeURIComponent(pairIdToBeSent, true));
                                        
        
        alert("call handle response");
        
 
        xmlHttpObject.send(null);       
} // end function

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Here's what I found. It's very weird:

Keep testing for a while and then the "Unknown Runtime Error" happens here:
document.getElementById('speedResultTable').innerHTML='<tr><td><img src="img/ajax-loader.gif"></td></tr>';

Then I tried removing the "tr" and "td":
document.getElementById('speedResultTable').innerHTML='<img src="img/ajax-loader.gif">';
Suddenly the problem disappear.

I put the "tr" and "td" back just out of curiosity. There's no more problem.
I'm confused. =(

hielo,
thanks for your explanation.