I think i may have found what is causing the problem.
I have been keeping an eye on cron and the problem always starts at 23.59 hrs, so i removed the following entries from cron for a few days and all was OK until i replaced them.
59 23 * * * /root/ispconfig/php/php /root/ispconfig/scripts/shell/ftp_logs.php &> /dev/null
59 23 * * * /root/ispconfig/php/php /root/ispconfig/scripts/shell/mail_logs.php &> /dev/null
59 23 * * * /root/ispconfig/php/php /root/ispconfig/scripts/shell/cleanup.php &> /dev/null
I think it could be a problem with ispconfig php but i don`t know what