[Dovecot] Index corruption

[Gregory Bond]

I'm getting pretty constant index corruption on (admittedly large) 
Maildir folders.

I'm running Thunderbird 0.7.3 and Mutt as my IMAP clients (on FreeBSD).  
Server is Dovecot 0.99.10.9
configured thusly:
    ./configure --disable-ipv6 CC=gcc-3.3 CXX=g++-3.3 
--prefix=/opt/dovecot --without-pop3d
on a Solaris 2.8 box.  Maildirs are NFS-mounted, but the index files are 
on local UFS filesystem:
    default_mail_env = maildir:~/Maildir:INDEX=/export/01/imapd/%u
MTA is Postfix, LDA is procmail delivering to several Maildir folders.

There will be 2 (or more) dovecot instances running for this user (me) - 
several from Thunderbird and one from the BIFF client (icewm).  Turning 
off the BIFF client doesn't seem to help.

Symptoms:  when using Thunderbird, and deleting a msg or moving it to 
another folder, Thunderbird will sometimes not display the next unread 
message, but just sit there.  Changing folders works, but changing back 
to the previous folder (or quitting/restarting Thunderbird) only shows 
the first N messages (N is about 500, in a Maildir of about 6000, and it 
is the SAME email that is "last" every time.)  This happens with 
multiple folders, but (usually) only folders that are delivered to by 
procmail (this may be related to the problem or a coincidence in that 
only those folders have unread email!)

Accessing the mailboxes using MUTT via IMAP shows the same truncated 
message list.

(I'm the only user using dovecot as I want Maildir, the other users have 
uw-imap, so dovecot is listening on port 4343)

Closing all imap clients, then removing the .index.* files from the 
appropriate /export/01/imapd/ directory, then restarting the imap client 
causes the missing messages to reappear (after a suitable wait for the 
index files to be recreated).

It really looks like locking or truncation issues in the index files....

Any clues? Debugging hints?  This is happening several times per day so 
I can get lots o' debug data!

Greg.