Outlook 2K Calendar currupt

We have 5 VP level users that have been experiencing issues when opening their mailboxes. When clicking on an box, outlook locks up (uses 99% of the processor) have to end process on it to close it. We have been running the clean mailbox tool fromthe exchange adminstrator and then having them reconstruct the imformation form the deleted items. We can not figure out what is causing this to occur. Has anyone seen this? It seems to only be a few of the users at this point and always seems tobe the calendar that is causing the lockup. Any help on this would be great. All users have laptops, Windows 2000, and a Blackberry handheld.