Ok I think that the problem is an intermittent USB connection on one of the data lines.
Why ? Nuvi 1xxx reboots when data connection to usb is lost (some other Nuvi's switch off )
Use Another Cable (Loly has already suggested)
Clean all USB plugs with alcohol and brush or contact cleaner spray.
Open Nuvi and check soldering and condition of usb female plug on main pcb.