Unexpected periodical Wi-Fi speed drop

Hi all,

I am encountering a periodic (each 1 second) Wi-Fi transmission speed drop 
(from 54Mbps to 6Mbps) during an iperf between two mesh portals. 
The speed stays stuck to 6Mbps during about 300ms and then recovers its 54Mbps
rate. About 1 second later this happen again.

The Wi-Fi speed drops from 54Mbps to 6Mbps without any intermediate speed nor
frame loss. 

My channel is clear from any other Wi-Fi devices disturbances.

My system is as follow :
                    __  _                             __  _ 
      192.168.2.41 [__]|=|    Wireless devices       [__]|=| 192.168.2.38
 1c:75:08:0b:76:52 /::/|_|    are close to each      /::/|_| 08:00:27:6e:54:11
                      |       other (about 15cm)        ^
              Ethernet|                                 | Ethernet
                      |      |_|_|           |_|_|      |
                      '---->[____°]         [____°]-----'
                        Mesh Portal         Mesh Portal
                  90:A4:DE:21:4F:11         00:1B:B1:58:F6:DD

Kernel : 2.6.33.7
Compat Wireless : 2012-06-14 
Radio chipset : AR9160 (ath9k)

Please find attached a wireshark wireless  trace of the drop itself 
(between packets 6 and 7).

Can you give me some clue how to work it out ?
Thanks in advance,

Best regards,

Cédric DEBARGE | ACKSYS Support Team
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