Having the same issues as the topic starter. Unstable connection behind the brick wall. With previous Dir-825 openwrt never had such problems and Dir-825 has 2dB antennae while RB2011UAS-2HnD-IN has 4dB.
Tried to lower signal levels to 17dB. No chance connection is unstable too. It looks like RouterBoards receiver just doesn’t see/listen WiFi device that is connected to it after some time of operation. Signal level on the phone or notebook is good but on RB2011UAS-2HnD-IN - bad. What can we do?
Also this is the same problem as described here http://forum.mikrotik.com/t/radius-server-not-working-in-2-8-11/127/1
So at least three guys here have that problem… Tell me what to do and I will do my best to localise the source of the problem!
Also the same issue I beleive is: http://forum.mikrotik.com/t/rb2011uas-2hnd-wireless-ping-loss/66157/1
PS: That issue was on 5.21, 5.23, 5.24, 5.25 and still remains on 6.0final.