Thanks for the several messages on and off list.
I've since realised that my random deletion prob is not in my new version of Eudora (since the same thing, it turns out with a bit of experimenting, can happen with the old one), but in my new keyboard, which is slightly smaller. And I hadn't tried looking in the Trash for accidently deleted text (d'oh), which will probably save my ass when my fingers do the jaywalking during the retraining interval... (I *had* tried Control-Z, as I mentioned, which didn't do the trick, so I stupidly assumed the text was gone for good.)
Thanks for the tips, everyone.
Cheers, Damien