Sudden problem with root password

["David Gaudine" <david@annette.concordia.ca>]

I just telnetted to my potato system and tried to su to root.
(I know I should be using ssh, but it's my ssh problem that
I was trying to fix, thanks Brad.)  I got the message
   su: incorrect password
Surely I didn't forget my own root password, but just in case,
I went physically to the system which already had a window
logged in as root, and changed the root password.
I still can't SU using either the old or new password.
I hope somebody can tell me what's wrong before the next
power failure.  (i.e. while I'm still logged in and don't have to
work from a rescue disk.)