I have a Raspberry Pi running PiAware headless (no monitor/keyboard/mouse) that I moved from wired ethernet to wifi, and every few days it becomes unreachable over wifi, but the activity lights appear to show it’s still up.
Looking through the syslog entries, around the time it appears to drop off the network there is this error that starts repeating:
Aug 31 00:00:09 piaware ntpd[14162]: error resolving pool 3.debian.pool.ntp.org: Temporary failure in name resolution (-3)
This post appears to see the same thing, and there’s a suggestion to turn off the power save function on the wifi:
sudo iwconfig wlan0 power off
I’ve given this a go and will monitor over the next few days.