Overlapping horizontal menus

Hello,

Working on a new site, with a different menu scheme.

I have this idea for a menu that I thought was going to work great, but now is not working great.

Each post has two categories, first name and last name, then 26 child categories. I would like the menu at the top to say “first name” then below this, have the letters of the alphabet. Then say last name and have another set of the alphabet.

It works, for the first few letters, then I get this crazy overlap. Any suggestions on how to make this work reliably?

See the link for a clearer description.

http://www.identifyartistsignatures.com/

Thanks,
mark