Are you using ChannelFly or SmartRoam for sticky clients? In my experience I've found those two features to be the biggest culprit in unexpected client disconnects. For troubleshooting, I'd recommend setting some static channel assignments, and disabling any sticky client roaming workarounds you may have turned on (SmartRoam is off by default, but ChannelFly may be on by default depending on your configuration settings in the initial setup)