Outlook 98-Why does email message freeze

We have a few users who are experiencing not being able to open up their email messages in Outlook. It’s not just one particular email, it’s several. Outlook just freezes up with no specific error message. We usually have to do a ctrl alt del in order to end the task. We have run the inbox repair tool, removed the services and re-added them…and still it freezes up? Help!!!!