If you use one of my NC's with memory boost it should work just fine.
I do agree that some XL models can be a pain to update but in general there should be no problems.
Try a clean install on an empty device.
Best option is just the plain NC and check if it works, you should get a "no maps..." warning.
After that add the map and use FastActivate to activate the map, the Navcore is already patched (if you use mine).
If your bootloader is still below 5.5112 it might also be a cause for problems with the V9 Navcores.
Most reboot problems are related to old stuff kept on the device before updating.