Strange: piwik sometimes track, sometimes tracks not goals(td)

i have a stupid problem in virtuemart 2 (joomla online-shop component) - my goals are not tracked reliable. ~40 percent orders are definitely not tracked as goals .
In my old piwik-version (must have been next to last) I tried JS tracking by page-title and part of uri on confirmation page. this page refers to paypal after 2 seconds. So i thought: "ok, perhaps this is the problem: 2 seconds is not enough to track reliable. I simply track goals one page before, onSubmit.
Today in the morning i updated piwik to Piwik 1.11.1 and put following code in the page:

jQuery(document).ready(function($) {
$('span#buyButton a').attr('href', '#');
$('span#buyButton a').click(function(event) {

…and before :

<script type="text/javascript">
      var _paq = _paq || [];
      _paq.push(["setDocumentTitle", document.domain + "/" + document.title]);
      _paq.push(["setCookieDomain", "*"]);

      (function() {
        var u=(("https:" == document.location.protocol) ? "https" : "http") + "://";
        _paq.push(["setTrackerUrl", u+"piwik.php"]);
        _paq.push(["setSiteId", "1"]);
        var d=document, g=d.createElement("script"), s=d.getElementsByTagName("script")[0]; g.type="text/javascript";
        g.defer=true; g.async=true; g.src=u+"piwik.js"; s.parentNode.insertBefore(g,s);
<script type="text/javascript">
        function logGoal(revenue){
            try {
                _paq.push(['trackGoal', 1, revenue]);
            catch( err ) {}
<noscript><p><img src="" style="border:0" alt="" /></p></noscript>

I changed the goal to manual triggering an then quick-tested: my visit and the goal was tracked correctly.
But now in the evening i looked again: again some orders were not tracked. I really ask myself why, the button has to be klicked for every order. nothing special in the order: the whole visit was tracked correctly.
I hope s.o. has a tip what i could test next.

Bye, Mojo

Update: I noticed, that in my last config Firefox-Orders are not tracked. I added a timeout for submit in onclick:

            $('span#buyButton a').click(function(event) {
                setTimeout(function(){ $('#checkoutForm').submit(); },1000);

My firefox test was successful, i am curious if everything works now.