Code is kind of messy, you should really think on how to better structure your CSS.
First solution that comes to my mind :
Remove the positions:absolute in the #navigation #menu-primary ul , and add :
.menu-item {
display: inline-block;
}
This should do the trick.
You should also avoid using ID’s and nesting so deep, as this is very hard to override in the future.
Thanks to https://stackoverflow.com/users/2455162/greatteacheronizuka