Menu bar in emacs (mostly) stops working

I'm not sure how to reproduce it, but sometimes after running fine for a while, the menu bar in emacs stops working except for the Apple and Emacs menus. The others (e.g., File, Edit, Options) all light up when I click on them, but the menu does not appear. I've moved all the windows out of the way, so it's not that the menus are getting obscured.

If I change buffers so there are new modes (and minor modes) active, the menu names change appropriately, but they still don't work.

I also tried creating a new frame but that didn't help.

The only solution I've found so far is to quit & restart emacs.

This is with Emacs 24.4 (MacPorts build) on Mavericks.