Users cannot access to their imap-folders

Users cannot access to their imap-folders.
Sometimes (once or twice a week) users complains that they cannot connect to their mailboxes and read emails. At the same time I see such records in maillog:
"Apr 10 15:01:01 mailserv postfix/lmtp[22843]: D64651001D37: to=< [email protected] >, orig_to=< [email protected] >, relay=mailserv.example.com[/var/lib/imap/socket/lmtp], delay=300, delays=0.13/0/0/300, dsn=4.4.2, status=deferred (conversation with mailserv.example.com[/var/lib/imap/socket/lmtp] timed out while sending MAIL FROM)».
It means mail queue increases
Also I find out lots of "PID-files" in /dev/shm/cyrus-imapd/proc which don't contain info of linked imap-folder. For example: 
"pc-user.example.com [192.168.0.11] user"
It's unhealthy situation and I don't understannd what reason is. In order to help users I just kill their IMAP-process and make reconstruct mailbox. Please help me with investigation of the problems.
My setup is:
--------------------------------------------
Static hostname: mailserv
Icon name: computer-vm
Chassis: vm
Machine ID: cbb779ad0b103a10ec75c10c452e0f71
Boot ID: c31a27599dc84971902e6c70dca26c18
Virtualization: kvm
Operating System: CentOS Linux 7 (Core)
CPE OS Name: cpe:/o:centos:centos:7
Kernel: Linux 3.10.0-514.6.1.el7.x86_64
Architecture: x86-64
--------------------------------------------
name : Cyrus IMAPD
version : v2.4.17-Fedora-RPM-2.4.17-8.el7_1 d1df8aff 2012-12-01
vendor : Project Cyrus
support-url:  http://www.cyrusimap.org
os : Linux
os-version : 3.10.0-514.6.1.el7.x86_64
environment: Built w/Cyrus SASL 2.1.26
Running w/Cyrus SASL 2.1.26
Built w/Berkeley DB 5.3.21: (May 11, 2012)
Running w/Berkeley DB 5.3.21: (May 11, 2012)
Built w/OpenSSL 1.0.1e-fips 11 Feb 2013
Running w/OpenSSL 1.0.1e-fips 11 Feb 2013
Built w/zlib 1.2.7
Running w/zlib 1.2.7
CMU Sieve 2.4
TCP Wrappers
NET-SNMP
mmap = shared
lock = fcntl
nonblock = fcntl
idle = idled
Server Wide:
freespace: 1820105352
------------------------------------
imapd.conf
------------------------------------
debug: no
commandmintimer: 2
configdirectory: /var/lib/imap
partition-default: /var/spool/mail
proc_path: /dev/shm/cyrus-imapd/proc
mboxname_lockpath: /dev/shm/cyrus-imapd/mboxname_lock
duplicate_db_path: /dev/shm/cyrus-imapd/duplicate_db
statuscache_db_path: /dev/shm/cyrus-imapd/statuscache_db
allowplaintext: yes
allowallsubscribe: yes
allowusermoves: yes
imapidlepoll: 10
idlemethod: idled
imapidresponse: yes
admins: cyrus
lmtp_admins: cyrus
singleinstancestore: yes
duplicatesuppression: yes
reject8bit: no
pass8bit: yes
munge8bit: no
lmtp_downcase_rcpt: no
sieveusehomedir: false
sievedir: /var/lib/imap/sieve
duplicate_db: skiplist
mboxlist_db: skiplist
postmaster: postmaster
postuser: shared
lmtp_over_quota_perm_failure: yes
lmtp_strict_quota: yes
quotawarn: 98
sendmail: /usr/sbin/sendmail
hashimapspool: 0
sasl_pwcheck_method: saslauthd
sasl_mech_list: PLAIN LOGIN
tls_cert_file: /var/lib/imap/SSL.crt
tls_key_file: /var/lib/imap/SSL.key
tls_ca_file: /var/lib/imap/CA_SSL.crt
tls_ca_path: /var/lib/imap
tcp_keepalive: yes
tcp_keepalive_cnt: 3
tcp_keepalive_intvl: 10
tcp_keepalive_idle: 180
temp_path: /tmp
telemetry_bysessionid: yes
timeout: 40
imapidletimeout: 0
expunge_mode: delayed
expunge_days: 30
deletedprefix: DELETED
delete_mode: immediate
flushseenstate: 1
-------------------------------------------
cyrus.conf
-------------------------------------------
START {
recover<----->cmd="ctl_cyrusdb -r"
idled><------>cmd="idled"
}
SERVICES {
imap cmd="imapd" listen="imap" proto=tcp4 prefork=5
imaps cmd="imapd -s" listen="imaps" proto=tcp4 prefork=10
pop3 cmd="pop3d" listen="pop3" proto=tcp4 prefork=3
pop3s cmd="pop3d -s" listen="pop3s" proto=tcp4 prefork=3
sieve cmd="timsieved" listen="sieve" proto=tcp4 prefork=0
lmtpunix<---->cmd="lmtpd" listen="/var/lib/imap/socket/lmtp" prefork=1
..
}
EVENTS {
checkpoint<-->cmd="ctl_cyrusdb -c" period=30
delprune<---->cmd="cyr_expire -E 3" at=0400
tlsprune<---->cmd="tls_prune" at=0400
squatter cmd="squatter -i" at=0600
}
--------
Best regards, Anton
2017-Apr-11

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