You banner has a z-index of 5. Your #nav has no z-index assigned. Can you give banner a (1) and assign z-index of (2) for #nav?
I see tons of questions from hundreds of users in this forum. Most them post HTML but never a link to an actual working model of the issue in question.
Not sure about others, but I don't have time to copy HTML and create an index page to view this issue.
Please post a link to your issue and I am sure your topic will receive better answers and no questions.