Tomtom rebooting problems
Here are some basic tips on how to solve these problems, if a hard reset (hold reset for 30s) and the clear flash tool don't work:

1. rebooting after a map update or changes to some map related files like POI's.

It is a common problem caused by the file "mapsettings.cfg", when moving it from an old map folder to a new map folder.
To get a correct version of this file without loosing your favorites connect your Tomtom to
Tomtom Home and use "operate my Go" to manually add a favorite - you can choose any address you like.
Exit and restart your TomTom, which now has a rewritten "mapsettings.cfg" file.
If everything works fine, you can delete the newly added "dummy" favorite.

2. rebooting after a Navcore update.

Make sure it is not related to the "mapsettings.cfg" (simply rename the file to test it and change the name back if it was not the cause).
Start over by doing a clean install of the Navcore :
Delete the files and folders of the Navcore and only leave the map folder on your device - I assume you did make a proper backup of your working system as mentioned in the Navcore section.
Copy all files and folders for your new Navcore on your device and start it.
If you still get a rebooting problem you might want to try a different Navcore.

3. rebooting problem without any updates made or after a shutdown caused by an empty battery.

As a last resort, connect your Tomtom to an original Tomtom car charger and turn it on.
I have not figured out yet why, but I can confirm, from my own experiences, that it can solve a reboot problem if all other options won't work.

Hopefully these simple instructions will help you.