Hi there,
There are a couple of things that may be happening. If the issue only presents it's self on you MacBook Air and not on any other devices (phone, tablet, other computers), then there may be a setting that is causing it. Take a look at the article troubleshooting steps for that issues.
Wi-Fi: How to troubleshoot Wi-Fi connectivity
http://support.apple.com/kb/HT4628
If the issue is network wide and occuring on multiple devices, then you may need to reset your Base Station to factory settings. The article below covers that process.
Resetting an AirPort Base Station or AirPort Time Capsule FAQ
http://support.apple.com/kb/ht3728
-Griff W.