Two Questions:

1)  Is this the Solpart Menu that is used for the standard installation, or is this the new DNN Menu control?

2)  Can you post an image that shows the behavior.

It appears a workaround that I put in for the Mac Safari browser is causing this issue.  I will need to rework it.  In the meantime, open up your dnn.controls.dnnmenu.js file and find the positionMenu function. Change the first couple lines from

positionMenu: function (oMNode, oMenu)
{
  var oPCtl = this.getChildControl(oMNode.id, 'ctr');
  if (oPCtl.tagName == 'TR' && oPCtl.childNodes.length > 0)
    oPCtl = oPCtl.childNodes[oPCtl.childNodes.length-1]; //fix for Safari and Opera... use TD instead of TR

to

positionMenu: function (oMNode, oMenu)
{
   var oPCtl = this.getChildControl(oMNode.id, 'ctr');
   if (dnn.dom.browser.isType(dnn.dom.browser.Safari))
   {
      if (oPCtl.tagName == 'TR' && oPCtl.childNodes.length > 0)
        oPCtl = oPCtl.childNodes[oPCtl.childNodes.length-1]; //fix for Safari... use TD instead of TR
   }

Note:  Safari users will still have this issue until I fix it for real.