Is this an AC access point? What FW? Is it standalone?
It sounds really similar to some issues I had when I first got my R700, where broadcast (Sonos) and multicast (Bonjour) packets were unreliable on 5GHz after some hours of uptime.
I worked around it by adding a controller, which made the problem magically go away, and I got too busy to hunt down the original issue. I read the 9.8.2 MR release notes had references to fixing a broadcast issue on AC AP's in standalone mode, so perhaps this is resolved in 9.8.2?