I've got 5 iPod nano devices in my household. I've tried them all in my M5 and they all have the random "disconnect/connect" issue, EXCEPT for my daughter's nano. I have no idea why her nano works, and all our other nanos have the same disconnecting issue. I've checked the cable, the software version, etc. I've even restored to nanos to original factory settings.
Needless to say, I've swapped nanos with my daughter!