problem displaying vertical menu css - css

Question

I've made a vertical menu using css. It's a menu with sub menus similar to this one:
http://www.dynamicdrive.com/style/csslibrary/item/suckertree-menu-vertical/
here you can see an example:
It work fine but when I click in one of the sub menus to see the information, the others sub menus disappear, that is the menu stay underneath the text. So if I want to change page by clicking in another sub menu I'm not able, I have to return to home.
Here is my css code:
#menu ul {
list-style: none;
margin: 0;
padding: 3px;
width: 100%;
font-size: 14px;
}
#menu h2 {
color: white;
background: #9370D8;
padding: 4px;
text-align:center;
font-size:15px;
width: 100%;
display: block;
}
#menu a {
color: black;
background: white;
text-decoration: none;
display: block;
font-weight: bold;
width: 100%;
padding:4px;
}
#menu a:hover {
color: black;
background: #eee;
}
#menu li {
position: relative;
}
#menu ul ul ul {
position: absolute;
top: 0;
left: 100%;
width: 100%;
}
div#menu ul ul ul,
div#menu ul ul li:hover ul ul
{display: none;}
div#menu ul ul li:hover ul,
div#menu ul ul ul li:hover ul
{display: block;}
and html code:
<div id="menu">
<ul>
<li><h2>Browse</h2>
<ul>
<li>Districts</li>
<li><a href="/browse/time/" >Time</a></li>
</ul>
</li>
</ul>
<ul>
<li><h2>Analyze</h2>
<ul>
<li>Co-occurrence
<ul>
<li><a href="/analyse/co-occurrence/percentage" >Percentage</a></li>
<li><a href="/analyse/co-occurrence/regions" >Regions</a></li>
</ul>
</li>
<li>Geographical
<ul>
<li>Districts</li>
<li>Citizenship</li>
</ul>
</li>
</ul>
</div>
For example I would use the link above. If I click on sub item 2.1 from folder 2, I will see some page with information.
Now I want to see the sub item 1.1 from folder 1, but my problem is when I click in one of the sub menus I'm not able to see the sub item 1.1, so if I want to click in sub item 1.1 I have to return to the main page
the problem is the following:
Any help would be appreciate :)
Thanks!

Answer 1

The problem was the z-index.
I read the tutorials and the front page had an z-index: -1;
and the menu had a z-index: 1;
So the following doesn't work, you have to set something like this:
body
{
z-index: 1;
}
sub-menu
{
z-index: 999;
}