I have a user whose previous PC was sluggish (in general), so I updated him with a relatively new model (Core 2 Duo, 2 GB RAM, 250 GB HDD, 256 MB Video Card). His email was server-based, but was using Outlook's cached exchange mode on his old PC. He has a very large Mailbox (7,701,304 KB & 119,804 Total Items), which is probably why he was using cached mode.
However, his new PC Outlook is setup to access his email through the standard, server-based ost (not cached exchange mode). This may be the cause for his problem (Can't See or Access Contents of Mail Folders Randomly). It affects folders randomly (possibly due to a mailbox store or memory cache maximum size being reached...?) The temporary workaround is to have him close Outlook and re-open it. After that, he can access those folders.
Is there a best fix for this that avoids the cached mode, or is that the only fix?
(As you are probably guessing, the user would not be willing to archive or separate his mail into a pst file, and can veto those suggestions.)
It's possible that the mailbox is corrupt. Any errors on backup? Another possibility is that his Outlook profile is corrupt.
Deleting and recreating the Outlook profile is the easiest thing to try. However, if you're getting errors on the backup trying to access his mailbox, then his mailbox is probably corrupt. In which case, one solution would be to export his mailbox using exmerge, delete then recreate the mailbox, and reimport the mail.