Why operate in 11b? use G if possible. dont rely on nv2 CCQ for your performance gauge, use real world testing instead.
you can use google earth ruler to find the exact distance.
if you give me coordinates at each end and antenna height at each end i can plug them into
radiomobile for you to help you more.
this post is informative, and will help you.
http://forum.mikrotik.com/t/solving-20km-wireless-link-issues/30025/1
also, it sounds like you are experiencing propagation effects, more notably a phenomenon called known as “thermal ducting”