Make sure all your network devices / computers are synced via NTP, or at least accurate to the second.

Hardwire the Pi to the network. Start a verbose traceroute and ping loop on a device on WiFi, as well as the Pi. Once it fails, observe the lights on your router/wifi device(s). Note time to the second of the start and finish of the outage, and the behavior of the lights. Check the results of your traceroutes/pings. Start digging through the logs of each network device.

Also, you can never quite rule out power, even if the lights stay on. A power conditioning UPS is useful here.