Occasionally while I work, without any special provocation, I lose connectivity via Wi-Fi. Other devices connected to the same wifi network have no interruption, and the problem gets resolved once I reboot my MacBook Air, so it's definitely a local problem.

Observations:

• The Wi-Fi symbol in the menu indicates that I'm still connected, but apps can't actually connect neither to the Internet nor to other devices in the LAN.
• I can't connect to an alternate Wi-Fi network (e.g. Wi-Fi tethering via iPhone).
• I can connect to the Internet via iPhone USB tethering but this seems to only work some of the time.
• Only a reboot solves the problem but a regular restart gets stuck on a grey screen with rotating wheel (after all applications have closed) and I have to do a hard reset.

How should I go about troubleshooting this? It used to happen very rarely but now is becoming more frequent (approaching once every 2-3 days on average).