I had problems like that until I upgraded my garmin USB drivers on my computer for some reason. It would just not be detected by my computer at times and I would have to fiddle with it endlessly. Or sometimes when it was detected, later on I would come and find it somehow stopped charging and had run the battery down trying to reconnect.
Once I upgraded the USB drivers, all of those problems went away for me. Not sure if it is the same issues you are having, but a 3 minute thing to try to see perhaps?
Garmin: USB Drivers Updates & Downloads