you might try:
/usr/bin/tvheadend_restart.sh
#!/bin/bash
sleep 5
/etc/init.d/tvheadend restart
chmod 777 /usr/bin/tvheadend_restart.sh
/etc/sundtek.conf
device_attach=/usr/bin/tvheadend_restart.sh
also check /var/log/mediasrv.log
maybe something accesses the analog TV tuner and prevents tvheadend to access the DVB tuner when restarting it.
udev triggers some analog TV detection with some linux systems, this will block DVB access until it is done (all that happens within a few seconds only), but as mentioned /var/log/mediasrv.log will show up what's going on.