[Dovecot] dovecot - outlook users keep getting locked out of imap

Greetings:

My outlook users accessing mail via dovecot's imaps have been consistantly getting locked out of their email. It happens about once a week per outlook user. It appears to happen when the are cliking on emails with attachments. When this happens, there are two imap deamons running for that user. I kill both deamons and things start working again.
Squirrelmail, thunderbird and pine users haven't had this issue. I'm running a version of dovecot dated dovecot-stable-20050610.

All user accounts have their mail on nfs mount points. The INBOX directory is local to the imap server but we have a smtp server that gets this data via nfs. Below is the dovecot.conf info:

base_dir = /var/run/dovecot/
protocols = imap imaps

ssl_disable = no
ssl_cert_file = /usr/local/imaps/certs/dovecot_cert.pem
ssl_key_file =  /usr/local/imaps/certs/dovecot_key.pem

disable_plaintext_auth = yes
login_dir = /var/run/dovecot/login

login_process_size = 128
login_processes_count = 9
login_max_processes_count = 256
login_max_logging_users = 512
verbose_proctitle = yes

mail_extra_groups = mail
default_mail_env = mbox:~/mail/:INBOX=/var/spool/mail/%u

mmap_disable = no
lock_method = fcntl

mbox_read_locks = fcntl
mbox_write_locks = dotlock fcntl

mbox_lock_timeout = 300
mbox_dotlock_change_timeout = 30
mbox_dirty_syncs = yes
mbox_very_dirty_syncs = no

protocol imap {
  imap_client_workarounds = outlook-idle
}

protocol pop3 {
}

auth default {
  mechanisms = plain
  passdb = pam
  userdb = passwd
  user = root
}

Your assistance is greatly appreciated.

Thanks, Anthony