Maxing out the top menu - unexpected limit creates related bug

I have a static database site that I am rebuilding from an older technology. I seem to have hit a limit on how many items total (all levels) can be included in a top menu. When I add a 91st item, it chops the last item off the menu list.

At the same time as it chops off the last item, it loses the definition of what menu to use for customized top navigation.

It's a completely repeatable bug -- I can break the menu setup at will.

This is a problem -- I was hoping to add another 10-20 items under my current structure.

See http://bluerose.karenlmyers.org/wp/

There used to be a Contact on the top menu to the far right, but it's item number 91. (If I move Contact earlier in the menu structure, it chops off the last item in the new menu instead.)