IPod Nano 6th gen has garbled sound only at full volume?

I'm using the Nano 6th generation with Bose Quietcomfort 15 headphones. For 6 months all was fine, then the audio become completely garbled on all tracks. Tried reset, then restore without success. I tried using the regular connecting cable instead of the special Apple cable: no change. By accident, I discovered that if the volume setting was around 90% or lower, then all was fine. Eliminating the equalizer had no effect. All music is MP3.