We had a Nüvi that kept rebooting when it was plugged in. It kept loosing 12V power and rebooted every time it did. I checked it with a different cable, and it did the same thing.
I eventually figured out the female plug on the back of the unit was bad, and every time the wire wiggled a little bit, it lost contact. I ended up replacing the Nüvi with a newer model.
Unless you want a Dezl or the equivalent RV model, Nüvi models with lifetime map upgrades can be had for under $150 with some below $100.