your HTML looks fine, though try using intends more effectively... it doesn't improve html validation, but it does help when it comes to having to read through loads of your own code. for example:
<div id="menu">
<ul>
<li><a href="#">Menu 1</a></li>
<li><a href="#">Menu 2</a></li>
<li><a href="#">Menu 3</a></li>
<li><a href="#">Menu 4</a></li>
<li><a href="#">More.</a>
<ul class="subMenu">
<li><a href="#">Menu 1</a></li>
<li><a href="#">Menu 2</a> </li>
<li><a href="#">Menu 3</a> </li>
<li><a href="#">Menu 4</a> </li>
<li><a href="#">Menu 5</a></li>
<li><a href="#">Menu 6</a> </li>
<li><a href="#">Menu 7</a> </li>
<li><a href="#">Menu 8</a> </li>
</ul>
</li> <!-- this was a <ul> in your posted code, the <ul> will have made the conflicts you where talking about -->
<li><a href="#">Menu 1</a></li>
<li><a href="#">Menu 2</a></li>
<li><a href="#">Menu 3</a></li>
<li><a href="#">Menu 4</a></li>
</ul>
</div>
..i only spotted one error within this HTML which i put a comment against, (you originally had it as a <ul> tag, but needed to be a closing <li> tag)
with your CSS, under the "#menu ul li ul.subMenu" styles, you may want to add the rest of the opacity codes for wider browser support:
#menu ul li ul.subMenu {
opacity: .88; /* Standard: FF gt 1.5, Opera, Safari */
filter: alpha(opacity=88); /* IE lt 8 */
-ms-filter: "alpha(opacity=88)"; /* IE 8 */
-khtml-opacity: .88; /* Safari 1.x */
-moz-opacity: .88; /* FF lt 1.5, Netscape */
}
and within the same block of codes (#menu ul li ul.subMenu), i noticed you have set a rather large "top" value set..
#menu ul li ul.subMenu {
top:28px;
}
...this will leave quite a large gap from the main menu to the dropdown menu... once your mouse enters this gap your drop down menu will close due to the fact you will no longer be hovering over the <li> element, with this in mind you may want to reduce that "top" value, or perhaps add "padding" to "#menu li" to close the gap.
.. apart from that, the rest of your CSS looks fine (:
one other thing... you will need to make sure you have a correctly specified Doctype at the very top of your HTML document for IE to display all your styles correctly, if you haven't got one yet, use the following...
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"
"http://www.w3.org/TR/html4/loose.dtd">
if you need anymore help, or if i haven't explained something very well, just ask (:
good luck with your project