Hello,
The units works perfect, no more reboots.
I have found 2 options:
http://www.extremeaudio.de/gb/phonocar-5-146-spannungsreduzierer-interf-12-v-stabilisator-4sek-timer.html (59 Euro)
http://www.extremeaudio.de/de/navlinkz-ac-pwr-st-digitaler-12v-strom-spannungsstabilisator-power-booster.html (79 Euro)
The 59 Euro units works and also solves the acc drop during starting the engine.