You could add a min-width:
.menu-principal {
min-width:883px;
}
Then it won't go on top of each other. However there are too many options on the navigation bar, you might want to make it a drop-down box when it goes into a mobile media-query range. e.g. http://vagish.com (as you make the browser width smaller, it changes the navigation bar)
You could have a media query which takes place between 768px and 880px, and reduces the spacing between menu items, and this should be able to make it look fine until you hit the 768px media query.
This would work (along with the CSS above):
@media all and (max-width: 768px) and (min-width: 880px) {
.menu-principal {
padding:0px;
}
.menu-med .item a {
padding:0 .69em;
}
}