Simulate Mouse Over in Vimperator plugin

I'm attempting to write a Vimperator plugin to allow use of hints mode to simulate mouse over on drop down menus. I have the hints mode working and can correctly choose elements that have mouseover events attached. The problem is my function to simulate the mouse over is not working. This is what I currently have:

function SimulateMouseOver(elem)
{
    var evt = elem.ownerDocument.createEvent('MouseEvents');
    evt.initMouseEvent('mouseover',true,true,
        elem.ownerDocument.defaultView,0,0,0,0,0,
        false,false,false,false,0,null);
    var canceled = !elem.dispatchEvent(evt);
    if(canceled)
        alert('Event Cancelled');
}

The above code works for some pages but not for others. For example it doesn't work on AccuWeather. Any ideas how to simulate a mouse over that will work for most pages?