MacBook – WiFi problems with new MacBook Air 11″

Try using iStumbler to detect and identify neighboring and overlapping wireless access points (WAPs). It's likely an interference issue that can be solved with vigilance. You need to find out what channels your neighbor's WAPs are using, and then switch your router to use the least crowded channel you can find.

Ideally you want to use channels 1, 6, or 11. These are the channels that have the most leeway when it comes to other channels interfering with their signal. Wifi devices operating on a particular channel can interfere with other devices operating on channels up to 5 steps away. The amount of interference grows logarithmically with signal strength and channel proximity.

For example, if you are using channel 1 and your nearest neighboring wireless network is also using channel 1, you are receiving a large amount of interference. If you are on 1 and the other network is on 6, the interference is negligible. However, if you are on channel 1 and your neighbor is using channel 3, you are likely still on the receiving end of a huge amount of interference depending on their WAPs signal strength.

iStumbler will give you information about the signal strengths emanating from the other interfering WAPs, and from there you can infer the best channel to use. You may need to periodically perform the same task as your neighbors change their wifi channels.

Have you tried going to the Network preference pane in System Preferences, clicking on Advanced, then Wi-Fi, and deleting all the Preferred Networks? I seem to recall having the same problem a few years ago, and deleting the networks fixed it.