[Dovecot] Dovecot main process killed by KILL signal

[For@ll]

Hi,

I have a dovecot 2.0.7 installed on ubuntu 12.10 and often in dmesg I 
see "init: dovecot main process (7104) killed by KILL signal", I must 
restarted dovecot, because I can't access to mailbox.

Server hardware:
2x1TB disks
4GB memory
1xCPU 2,4GHz
HARDWARE RAID 1
Mailbox ~ 1k

dovecot.conf

# 2.1.7: /etc/dovecot/dovecot.conf
# OS: Linux 3.5.0-22-generic x86_64 Ubuntu 12.10 xfs
auth_mechanisms = plain login
auth_verbose = yes
default_client_limit = 1500
info_log_path = /var/log/dovecot.info
log_path = /var/log/dovecot.log
log_timestamp = "%Y-%m-%d %H:%M:%S "
mail_location = maildir:/var/mail/virtual/%d/%n/
maildir_very_dirty_syncs = yes
managesieve_notify_capability = mailto
managesieve_sieve_capability = fileinto reject envelope 
encoded-character vacation subaddress comparator-i;ascii-numeric 
relational regex imap4flags copy include variables body enotify 
environment mailbox date ihave
namespace {
   inbox = yes
   location =
   prefix = INBOX.
   separator = .
   type = private
}
passdb {
   args = /etc/dovecot/dovecot-sql.conf
   driver = sql
}
passdb {
   driver = pam
}
plugin {
   sieve_dir = /var/mail/virtual/%d/%n/sieve
   sieve_global_dir = /var/mail/virtual/sieve
}
protocols = sieve imap pop3
service auth {
   unix_listener /var/spool/postfix/private/auth {
     group = postfix
     mode = 0660
     user = postfix
   }
   unix_listener auth-master {
     mode = 0600
     user = vmail
   }
   user = root
}
service imap-login {
   inet_listener imap {
     port = 0
   }
   process_limit = 512
}
service imap {
   process_limit = 512
   service_count = 0
}
service managesieve-login {
   inet_listener sieve {
     port = 33919
   }
}
service pop3-login {
   inet_listener pop3 {
     port = 0
   }
}
ssl_cert = </etc/postfix/rssl/ssl.crt
ssl_key = </etc/postfix/rssl/ssl.key
userdb {
   args = uid=1003 gid=1003 home=/var/mail/virtual/%d/%n allow_all_users=yes
   driver = static
}
userdb {
   driver = passwd
}
verbose_proctitle = yes
protocol lda {
   auth_socket_path = /var/run/dovecot/auth-master
   info_log_path = /var/log/dovecot-lda.log
   lda_mailbox_autocreate = yes
   lda_mailbox_autosubscribe = yes
   log_path = /var/log/dovecot-lda-errors.log
   mail_plugins = sieve
}
protocol sieve {
   disable_plaintext_auth = no
}
protocol pop3 {
   pop3_uidl_format = %08Xu%08Xv
}

Any ideas what's wrong, where is the problem?