apple TV 2 issue: iTunes music playback stops after 15 seconds

This really is annoying:

I stream/airplay from my iMac (10.11.6) with iTunes (12.5.) to my AppleTV2 (6.2.1.)

Occasionaly, around every 50th song, the playback on my apple TV stops at 14 to 15 seconds into the song. It's every time exactly 14-15 seconds.

Press play again and it goes on.

No speciality on songs (ripped, purchased, downloaded - doesn't make any diference).

Did already all the usual - restarted appleTV, restarted the router (AirPort Extreme).

Any ideas out there?

regards

Harald