There is a JavaScript Bug in PTControls.js (/bea/alui/ptimages/imageserver/plumtree/common/private/js/jscontrols/LATEST) if the doctype of the portal page is set to HTML 4.0 Transition. The portlet menus will not be displayed beneath the portlet header, but in the upper left corner of the browser window. To fix this bug you have to add

if (this.popup.style.left == '') {
   this.popup.style.left = '' + left + 'px';
   this.popup.style.top = '' + top + 'px';
}

to the 'PTMenu.prototype.show' method in line 13688.