I had this exact same problem with my Montana 700i a couple of years ago. Without boring everyone to death with the excuses Garmin spewed out week after week, it was eventually confirmed by Garmin that the issue was the mapping software version conflicting with the device software. They said they could fix it if I paid them a few hundred pounds more for a different version of the software but as I was using the default software installed on the device at the time of purchase I declined their generous offer and instead opted for a refund.