Symptom:
CISCO891W-AGN-A-K9:  AP modules (Airconnect, designed by Cisco; Airforce 2, OEM-designed) goes to a failed state and hang up:

IOS Bootloader - Starting system.
Xmodem file system is available.
Bringing up flash file system: 
flashfs[0]: 168 files, 11 directories
flashfs[0]: 0 orphaned files, 0 orphaned directories
flashfs[0]: Total bytes: 31481856
flashfs[0]: Bytes used: 24268288
flashfs[0]: Bytes available: 7213568
flashfs[0]: flashfs fsck took 20 seconds.
Reading cookie from flash parameter block...done.
Base Ethernet MAC address: c4:71:fe:ef:0a:ce
Ethernet Interface initialize. 
Ethernet Interface initialize OK. 
TFTP initialize OK. 

The system is not configured to boot automatically.  The
following command will finish loading the operating system
software:

    boot

AP801 Boot Loader (AP801-BOOT-M) Version 12.4(10b)JDA3, RELEASE SOFTWARE (fc1)
Technical Support: http://www.cisco.com/techsupport
Compiled Sun 07-Jun-09 04:01 by prod_rel_team

ap:

Conditions:
Not all units fail with these steps:

1- The Airconnect Part could be accessed via session. 
2- The Airconnect Part could passed the 2.4G and 5G wireless link test. 
3- The received unit's AP has joined with WLC, and the WI-FI able to associated with the received unit's AP.  
4- Keep link for about 2 hours, AP reboot occured. 

When it fails, the AP is hung.