A loose USB port at the rear of a nüvi is almost always the result of briefly pulling the power cable. The force exerted by the cable on the USB port causes the post to become partially detached from the PCB (printed circuit board) inside the nüvi. It can easily be prevented. This is how I have secured the power cable to a nüvi 1440 some time ago on request by a friend who had experienced a loose USB port once before:
Disadvantage is that the power cable can not physically be detached from the nüvi anymore and will stay with it. If that is no problem, than this solution may be what you're looking for. All you need to do is carefully open up the nüvi, remove the PCB and drill two small holes in the rear cover at the right spot so the ty-wrap will fit through.