XTR won't exit sometimes

Hi,
I have a tiny problem with my XTR cronjob in my 1.8 environment. Cron is scheduled for every 5min to run xtr in the regular way:
crontab -e -u www-data
*/5 * * * * /usr/bin/php /var/www/xibo/bin/xtr.php > /dev/null 2>&1

Now it happens from time to time, that not every xtr job is terminated correctly and is hanging in the system (consuming DB sessions, RAM,…):

ps -ef | grep xtr

www-data 3237 3236 0 04:35 ? 00:00:00 /bin/sh -c /usr/bin/php /var/www/xibo/bin/xtr.php > /dev/null 2>&1
www-data 3405 3404 0 05:05 ? 00:00:00 /bin/sh -c /usr/bin/php /var/www/xibo/bin/xtr.php > /dev/null 2>&1
www-data 3564 3563 0 05:20 ? 00:00:00 /bin/sh -c /usr/bin/php /var/www/xibo/bin/xtr.php > /dev/null 2>&1

Of course I tried to redirect output of XTR into a log file. But there was simply nothing send to stdout or stderr.

Any idea what’s going wrong here or how to increase verbosity in XTR to better see what is going on?

Thanks for any help!
Best
Daniel