Can't add word to custom dictionary in Entourage

About 80% of the time, I can't add a "good" word to my Entourage
(shared) custom dictionary. In fact, Entourage doesn't recognize
words that I added when the spell checker was working properly. The
only thing that I have found that will fix it is to restart Entourage,
which is most often, entirely unacceptable, as I usually have multiple
windows open at the same time, that I would have to close. Besides,
restarting the program every half hour is a Windows fix, not a Mac
fix. I can only compose 2 or 3 emails after start up, before
Entourage forgets how to add a word to the custom dictionary. Here
are the conditions:

There is only ONE custom dictionary (shared) available in both Word
and Entourage.

Language of the custom dictionary is "none".

Location of custom dictionary is the same in both programs (checked at
time of failure).

Primary (not custom) dictionary is "US English".

At time of Entourage failure, I can open a document in Word and add to
the custom dictionary.

New install of Office X and OS X (neither is an upgrade).

Any ideas that I haven't tried, short of trying to find to a better
email program? Thanks.