I encountered the same issue regarding the dropdown links area for the navigation bar, the dropdown box would show then disappear when moving to hover over link, I experiened the problem only with IE, 7 & 8. Firefox, Google Chrome, & Safari did not seam to have this problem.
Found 2 ways to fix but only one is friendly to all browsers.
1) make image below navigation bar a background image using CSS (great for IE but not so great for other browsers which would not use the background image so to remidy this I placed image physically then used CSS to hide image if browser is IE on page load. Only problem I encountered was that IE8 would not display the background image and because of the CSS rule would not display the physical image, all other browsers worked great.
2) using CSS rules on page load strictly for IE only adjustments to the background dropdown. Width is of course is adjustable as needed or you can use transparent gif files in the menu.php file to ensure that the width of the box is the same for all links, place the gif file after the text but be sure to place the image file within the <a> tag. The actual fix for the dropdown falling behind is using the float rule, this at least worked accross all browsers.
The code used on page is below.
<!--[if IE]>
<style type="text/css">
#navWrapper #topNavigation ul#cssmw_home ul.level-1 > li > a {
float:left;
width:200px;
}
</style>
<![endif]-->
Good Luck!
