Following numerous other updates from Apple this week, the company has released a firmware update for AirPort Extreme Base Stations and Time Capsule devices which address problems when using the 5GHz band.
Old firmware to blame.
Tutorial: Fixing Wireless connectivity (including AirPort) problems: Dropouts, slow speed, more
AirPort Extreme Update 2007-001 Special Report: Dropouts (lapses in connectivity), fixes
More on wireless router dropouts (AirPort Base stations included): Try toggling firmware
Success with toggling firmware revisions for wireless router dropouts
Utilities Updates: TechTool Pro 4.5.2; Adobe Acrobat Pro 8.0; OmniWeb 5.5.1; PGP Desktop Home 9.5.1; more
Utilities Updates: Safari Extender 1.6.0; OmniWeb 5.5.1 beta 1; nova media iSync plugins 3.3.2; more
Downgrade AirPort Extreme firmware for repeated dropouts (lapses in connectivity)
AirPort connectivity dropouts after 2005-001 update