the only way I could fix it was to change the width: auto to width: 200px; in this rule in the nav.css
.nav li > ul
I played with width: 100%; but that did some funky things to the drop down. Maybe with it being your code, you can figure out how to make it work out.
I am using I.E tester and it works in that. You might have to play around with the values.
Also I noticed that in your css, you have many places where the last rule is not followed by a semi-colon. While the css does not care that you don’t put it there, it is considered best practice to always follow every rule with a semi-colon. This will save you problems later on if you should forget to add the semi-colon when you go back to add another rule.