Problem with mainmenu

Hi guys,
I’ve a little problem with my mainmenu.
As you can see here https://www.zerogradinord.net/vela-in-scatti/
or here https://www.zerogradinord.net/volvo-cup/ in the mainmenu I’ve some black line, but if you pass with the mouse on it you will see some menu voice. What I’ve to do to have all the line white like the first one or the one that is in the middle?

Down here you will find the Style.css block regarding the Horizontal menu code:

/* Horizontal menu */

#mainmenu {
clear: both;
width: 100%;
margin: 0px;
padding: 0px;
}

#mainmenu ul.level1 {
border-top: 1px solid #000;
border-bottom: 1px solid #000;
}

#mainmenu ul {
background: #000000 url(images/menubgzgn.jpg) center left repeat-x;
padding: 0 0 0 5px;
margin: 0px;
border-bottom: 1px solid #000;
}

#mainmenu li {
background url(images/menubgzgn.jpg) center left repeat-x;
display:inline;
line-height: 25px;
margin-left: -4px;
padding: 0px;
font-size: 0.9em;
list-style: none;
text-transform: uppercase;
}

#mainmenu a {
padding: 6px 9px 6px 9px;
text-decoration: none;
border-right: 0px solid #ffffff;
}

#mainmenu li.current a {
color: #000;
}