the issue iis in the styles for these 2 elements:
<div class='cssLO stmhINDEX_column_6_layout'>
<div class='column_6 cssLI stmhINDEX_column_6_design'>
although they have a z-index set, they do not have a position set. z-index must be used in conjunction with the position.
stacking order on the page is first determined by the position, then the z-index for that position. each position creates a layer, the z-index determines the stacking oder inside that layer. by not specifying a position, it is defaulting to absolute, which is a higher layer then the menu is using which uses the relative position, you should add position: relative; to those div styles where you are setting the position.
this is going to create another issue though, which is that IE cannot use the auto height setting, which you are using on those divs. For IE, you will need to specify an exact height for those elements.
there isn't any one element that is causing the problem, so it is going to be fairly difficult to pinpoint exactly what the fix is, you will need to do some considerable experimentation. I would suggest using IE conditional CSS on those elements to over ride the existing css. more on coditional css here: