[imp] IMP sometimes not listing messages in a mailbox (appears empty though there are messages)

[nashjunk]

I run a small hosting service and provide IMP for webmail access. I recently
migrated from IMP 3 to IMP 4 and two users have reported this same problem.
Sometimes, their inbox will display as if it's empty. In other words, in the
list of messages, there is nothing shown - though IMP will say "1-20 of 100"
(for instance). Clicking to the next page may display the next 20 messages
correctly - and then the next page may be blank too. Note that these are not
just completely blank pages caused by a PHP error - the full IMP sidebar,
header, etc display, but there are no messages. A picture is the best
explanation, so here is what I mean by screenshot:

http://img200.imageshack.us/img200/9940/impproblem.gif

Upon debugging this problem, I discovered that deleting certain messages
will fix this problem. There seems to be no rhyme or reason to which emails
cause the problem. For the first user who reported this problem, we purged
out all unneeded email and she was once again able to see all her messages.
A few days later the problem returned after receiving some more email.
Again, we purged some unneeded email and again it was fixed.

If you click on the various sorting headers (Date, Subject, etc) you can
sometimes get messages to display since different messages will be on that
page.

In other words, it appears certain messages trigger the issue and will cause
the listing to be blank if they are part of the listing.

I upgraded uw-imap from a 2002 version to the 2007e version with no luck.
PHP was upgraded to 5.2.10 from an earlier 5.2 version with no luck. IMP and
Horde were updated to the latest versions from earlier H3 releases with no
luck. At this point I do not know what else to try. Of note, IMP 2 does not
exhibit this behavior (I can get full listings of the trouble mailboxes in
IMP 2). Also of note, DIMP will simply say "No Messages" despite saying
"Inbox (244)" on the sidebar.

I would greatly appreciate any insight into this problem, as the only
solution I can offer my affected users right now is to use the older IMP 2
version (which obviously is not nearly as nice as IMP 3).

Program versions:
IMP 4.3.4, Horde 3.3.2, Apache 2.2.9, PHP 5.2.10, UW IMAP 2007e (built from
source RPM), RHEL 3 OS

Thank you!

- Alex