You can try upgrading your security measures to WPA2 Personal. Also, fire up AirPort Utility 5.6 and enable "Use Wide Channels". That AirPort is getting long in the tooth, and the new ones are worlds faster. However, if this helps you out then continue with it.
You can download the older copy from here. It will be under the wireless tab under "Wireless Options". Otherwise you can try resetting your Extreme with a paper clip and setting it up via 6.3.1 totally!