Using XHR to send large amounts of data via POST.

Question

Hello, I have an XHR that uploads large amounts of data to the server. This XHR dies intermittently and seemingly without pattern. Currently my code looks like this:

function makeFormData(data) {
    var formdata = "";
    for (var key in data) {
        formdata = formdata + "&" + key + "=" + encodeURIComponent(data[key]);
    }
    return formdata.slice(1);
}

function xhr(url, data, onsuccess, onerror) {
    var xhrequest = new XMLHttpRequest();

    xhrequest.onreadystatechange = function () {
        if (xhrequest.readyState != 4)
            return;

        if (xhrequest.responseText)
            onsuccess(xhrequest.responseText);
    }

    xhrequest.onerror = function (error_param) {
        onerror(error_param);
    }

    xhrequest.open('POST', url, true);
    xhrequest.setRequestHeader("Content-Type", "application/x-www-form-urlencoded"); 
    xhrequest.send(makeFormData(data));
}

This is in a Chrome extension. What am I doing wrong? How can I make the form multipart instead, to make the upload smaller? I have used Wireshark to trace the request and the it cuts off mid-send with many packet retransmissions (and never completes).

Please help me, this is driving me crazy.