The problem is that your device is not stateless, it has a lot of places which store information, and which you can't directly access. And if one firmware ****s up something so that the other misbehaves, you will end up with one wrong bit among megabytes of configuration data. This isn't just my dream, I've seen some 13xx devices creating cr acked up GarminDevice.xml after flashing an asian firmware, and NV cleaning not helping (There is something that does help, but then you loose your UnitID and some other values).
Btw, have you backed up your NV memory?