[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?