This issue only occurs in IE 7 and is also an issue in the Powerstore Demo.
At the top of the index.php change the following code:
@import url("css/common.css");
#outerWrapper #navWrapper {
padding-bottom:0px;
z-index:999;
}
to
@import url("css/common.css");
#outerWrapper #navWrapper {
padding-bottom:8px;
z-index:999;
}
Just change the padding-bottom from 0px to 8px
Hope that helps. If it still goes away you can go up to 10px