osticket hang-up

Using osticket 1.9.4.

Sometimes osticket stop working, there is no access to the panel possible. I have check the logs and found in the mysql slow query log:

# Time: 161111  9:57:18
# User@Host: osticket[osticket] @ localhost []
# Query_time: 1213.263497  Lock_time: 0.000163 Rows_sent: 0  Rows_examined: 837077
SET timestamp=1478854638;
SELECT DISTINCT COALESCE(B1.ticket_id, B2.ticket_id, B3.ticket_id, B4.ticket_id) FROM (
                SELECT object_type, object_id, MATCH (search.title, search.content) AGAINST ('Neue Bestellung' IN BOOLEAN MODE) AS `relevance`
                FROM `ost__search` `search`
                WHERE MATCH (search.title, search.content) AGAINST ('Neue Bestellung' IN BOOLEAN MODE)
            ) `search` LEFT JOIN (select ticket_id as ticket_id from ost_ticket
            ) B1 ON (B1.ticket_id = search.object_id and search.object_type = 'T') LEFT JOIN (select A2.id as thread_id, A1.ticket_id from ost_ticket A1
                join ost_ticket_thread A2 on (A1.ticket_id = A2.ticket_id)
            ) B2 ON (B2.thread_id = search.object_id and search.object_type = 'H') LEFT JOIN (select A3.id as user_id, A1.ticket_id from ost_user A3
                join ost_ticket A1 on (A1.user_id = A3.id)
            ) B3 ON (B3.user_id = search.object_id and search.object_type = 'U') LEFT JOIN (select A4.id as org_id, A1.ticket_id from ost_organization A4
                join ost_user A3 on (A3.org_id = A4.id) join ost_ticket A1 on (A1.user_id = A3.id)
            ) B4 ON (B4.org_id = search.object_id and search.object_type = 'O') LEFT JOIN ost_ticket A1 ON (A1.ticket_id = COALESCE(B1.ticket_id, B2.ticket_id, B3.ticket_id, B4.ticket_id)) LEFT JOIN ost_ticket_status A2 ON (A1.status_id = A2.id) WHERE ((A1.staff_id=5 AND A2.state="open") OR A1.dept_id IN (1,5))ORDER BY `search`.`relevance` LIMIT 500;

It seems to block all other processes. While this happens I run "show processlist;" and it shows:

63933 | osticket | localhost       | osticket | Query   | 1192 | Copying to tmp table         | SELECT DISTINCT COALESCE(B1.ticket_id, B2.ticket_id, B3.ticket_id, B4.ticket_id) FROM (

All other queries wait to get table lock. What can I do to prevent this hang-up?