Ctrl-Z and Ctrl-Y are broken

[TonyDaly]

John,

You say "I mention Ctrl-Z and not the UNDO function available in the Edit menu, or with a toolbar icon, not because it is only Ctrl-Z that is broken, but only because I only ever use Ctrl-Z for this. I do not know whether the other UNDO methods are also dangerous to use."

Can I suggest that you try out the menu items. If the menu items work it may be a problem with the mapping of the shortcut keys on your system, particularly if you have another application open at the same time that is taking priority. It may explain why it is intermittent.