Hi - I'm happy to help with your Mac issue. Please remember to Accept when we are completely finished and you are satisfied with my answer. We can try to rebuild this mailbox in the hopes that it's just an indexing issue. Close Entourage. Then re-open it with the Option key pressed. This brings up the Database Utility. Select Main Identity DB then do a rebuild database. Let me know how it goes.
When you are ready to resume, please let me know.