AirPort Extreme Drops Out Periodically (Every 2-3 Days) Requiring Unplug/Replug to Restart
Solution 1:
Make sure you don't have any 2.4Ghz cordless phones near your Airport Extreme. Some of this phone will tend to do what you describe when they interfere with your Airport Extreme, at least that used to happen to me with an Airport Express I had.
Solution 2:
The best solution I've found so far (and this is obviously a bit of a hack) is to put the Airport onto a light timer so it turns off and then on again at 4:45 am every day. This causes it to reboot and seems to end the problems, but it's at a time of day I don't find myself wanting to use the network.