Problem With 802.11n + chain 0

I have a Wireless link with RB411AH+R52HN+ 1-30dBi MikroTik Dish antenna foe each side .
for R52HN connect two MMCX pigtail that I only use one of them .
ccq >99% signal=-51 for 11 Km and NV2 + btest > 112mbps for send and >102mbps for re

My problem is :
After 3~5 week my link is down
My scan list is empty …
and when I change my wireless card it work for 3 week again.

I only use chain 0
mode : 5Ghz-OnlyN

do you use some band filters or you connect directly to the antenna?
You have only one wireless card in that board?
What TX-power you use, try to lower the tx-power.
Check what happened to the first wireless card:
http://wiki.mikrotik.com/wiki/Manual:Wireless_card_diagnostics

I check my card and N-type and Antenna …
I change first all pigtail because of rainy day …
second change my card … 3 ; routerboard …
but my problem is not solve …
today I change my antenna and change the antenna hight ( from 31m to 21m ) and I connect 2 antenna ( h&v ) and change my mini pci , Routerboard and pigtails with older one my problem is SOLVE I don`t now why ??? my problem is solve but I want to know why when I use link with not different (R52Hn,R52NM ) with one chain this problem is happen … I test it …
If you want I can send support.rif .
know I have install 6 802.11n wireless link with one chain (chain 0 ) until now I change 2 link which is use with one antenna with two antenna … and tomorrow I have to change the another …
It work , it work very good I can copy file from one windows OS to another one with about 94Mbps (office 2007 zip file about 509M about 44 second ) but only about 4~6 week .
I change my router OS 5 RC7,RC8,RC9,RC10 but not different .

please tell us more detailed what you did?
You use not chain0 and chain1 with 2 antennas connected?
Check the signal for chain0 and chain1.