MacFixIt has front page coverage of this problem. The workaround is to downgrade to AirPort Base station firmware to version 5.5.1. To quote MacFixIt, “Emerging as the most commonly successful solution for this issue is a downgrade of AirPort Base station firmware to version 5.5.1. Reverting (or updating to) this edition of the firmware has eliminated dropouts for a surprisingly high number users. The current AirPort Extreme firmware version, with which many users are experiencing dropouts, is 5.7.”