Matt Warner wrote me about a problem that occurs when you have the Menu Underline option (as used in Windows software) turned on in Excel 5.0a for the Macintosh. The result is that "submenus float at random vertical positions on the screen, making them totally unusable. The solution is to go to Tools, Options, and the General tab and turn off the menu underlining feature."
George Anderson wrote me a few days later with a very similar problem. I suggested trying Matt's cure - and it worked for George as well.