Q: AirPlay stops after iPhone device screen goes black

iphone6s with up to date software. 3rd gen appletv with up to date software. Both on same 5ghz wifi in my home. in safari, I'll start air playing an embedded video on a page in Safari on my phone and everything is fine. After a few seconds of playing the phones black "airplay" screen dims and still everything is ok. a few seconds later the phone screen goes black and the AirPlay stops. If I wake up the phone, I'm back in the black "AirPlay" screen on the phone and the AirPlay on the tv starts over. I've restarted the phone and the appletv with no change. Very frustrating. And ideas? Thanks in advance!