Binary package hint: gnome-panel
When GConf key /desktop/gnome/interface/gtk_key_theme is set to Emacs, main menu exhibits odd behavior. When invoked via Alt-F1, whereas pressing down arrow on the keyboard intuitively moves the selection from the main menu item itself onto the first entry below, that is not the case with Ctrl-N: it advances the selection from "Applications" to "Places", which is horizontal movement (for horizontally oriented menu, at least). Both Ctrl-N and Ctrl-P will move the selection horizontally, between "Applications", "Places" and "System", until either Ctrl-B or Ctrl-F is pressed: at that point the selection moves into the currently open submenu and things get restored to their normal, non-axis-swapped state.
Thank you for your bug. What version of ubuntu are you using? Why do you expect those actions to react this way?