5ghz ath wifi disconnecting

Two weeks ago my wifi starts to disconnect (or however one can name it) from the ISP AP. Having routerboard 133c with atheros 5312 atheros wireless card.

I tried to upgrade routeros to 2.9.50, tried to exchange wifi module, but it still happens.

If it happens rx-rate lowers to 6Mbps and rx-ccq disappears after a while. when I do
/interface wireless disable wlan1
/interface wireless enable wlan1
it works again. And if I let it be, it starts working after some (long) time.

I investigated, that there are another 2 networks on the same frequency (5500MHz) with ~20dBm lower signal and another 2 in ± 20 MHz range.

If I turn wireless logging on (adding wireless to /system logging), nothing appears in the log when it happens (so no disassociation or disauthentication).

It’s ~1km directly visible connection with 23dB parabolic antenna connected to ant-b.

Doesn’t anybody have any clues?

Some info available here:
http://www.fi.muni.cz/~xslaby/sklad/rb_problem/