I had exactly same issue with the DAC2. When I hit a light switch the music drops out. It happened with several digital cables. It's the dac. I called Bel Canto with this question and they said it's normal and happens because the dac is on the same circuit as the light switch.
But I can tell which digital cables you can use that sound very good and the dac is not affected when you use these cables:
Acoustic Zen Silver Bytes, Virtual Dynamics NiteII and Virtual Dynamics Master.
With the VD cables it never happened. With optical cables, it happens almost every time the lights are turned on or off.
But in any case, do not worry about it, it's annoying but nothing serious.
But I can tell which digital cables you can use that sound very good and the dac is not affected when you use these cables:
Acoustic Zen Silver Bytes, Virtual Dynamics NiteII and Virtual Dynamics Master.
With the VD cables it never happened. With optical cables, it happens almost every time the lights are turned on or off.
But in any case, do not worry about it, it's annoying but nothing serious.