Outlook *.pst problem.

We have a few associates complaing that their Outlook sometimes freezes on them. We have found that removing a *pst file from their Outlook folders can resolve this. The last one I removed was only 468 mbs; not close to the MS 2GIG pst limit. Of course, removing the file is impractical if they really use the *.pst file much. All patches and updates are up to date.
Any ideas welcomed.