IE6 SP1 Freeze on syncronous send

[SBGibson]

Hi everyone,
I'm developing a small ajax functionality and it works well in IE6 sp2, IE7 and Firefox.
It only have to call a webpage in background, nothing special.
With IE6 SP1 (I'm still investigating if it's some particular version) when I call Send the browser freeze, for a couple of minutes, after that timeout it goes on with the request. Investigating I've found that in the request header the "connection: close" item is not written so I think that httprequest wait for a close indefinetly (timeout).
If I configure IE to use HTTP/1.0 instead of HTTP/1.1 it works, probably cause 1.0 does not support multiple connections and auto-add the Connection:close header.
I updated to the last version of IE (SP1) and installed XMLHTTO 6 without any improvement. Some hint? I add some piece of code:

The ajax engine (I already tried forcing version 6 without success)

function getAjax() {
	var XHR = null;
	browser = navigator.userAgent.toUpperCase();
	if(typeof(XMLHttpRequest) === "function" || typeof(XMLHttpRequest) === "object") {
		XHR = new XMLHttpRequest();
	} else if(window.ActiveXObject && browser.indexOf("MSIE 4") < 0) {
		if(browser.indexOf("MSIE 5") < 0) {
			XHR = new ActiveXObject("Msxml2.XMLHTTP");
		} else {
			XHR = new ActiveXObject("Microsoft.XMLHTTP");
		}
	}
	return XHR;
}

The request function (I tried monitoring onreadystatechange and it never goes after 2)

	var ajax = getAjax();
	if(ajax) {
		ajax.open('get', '/test/background.aspx', false);
		ajax.setRequestHeader('connection', 'close');
		ajax.send();
}

request (in working browser appears also the line "connection:close")

GET /test/background.aspx HTTP/1.1
Accept: */*
Accept-Language: it
Referer: [URL unfurl="true"]http://testforge/test/test.htm[/URL]
Accept-Encoding: gzip, deflate
User-Agent: Mozilla/4.0 (compatible; MSIE 6.0; Windows NT 5.1; .NET CLR 1.1.4322; .NET CLR 2.0.50727)
Host: testforge
Cookie: ASP.NET_SessionId=hbmlltnz0tqb1wiymbq0ez55

Response

HTTP/1.1 200 OK
Date: Thu, 25 Oct 2007 10:34:40 GMT
Server: Microsoft-IIS/6.0
X-Powered-By: ASP.NET
X-AspNet-Version: 1.1.4322
Connection: close
Pragma: no-cache
Cache-Control: no-cache
Pragma: no-cache
Expires: -1
Content-Length: 10

Stevie B. Gibson