Looses connection

I have a similar but also different issue with my EX6200 (I have started its own thread about it - awaiting MOD approval). My EX6200 also in 5GHz fastlane mode, simply will not see my routers 5GHz channel, even though in range with good coverage and all other devices can connect fine. It also does not see it in standard 5GHz (none fastlane - repeater) mode. It has worked in 5GHz mode a few times (very rarely) and to be honest when it does it is actually very good and fast enough as that is the best way to connect hard wired devices to get the fastest link, but for me most of the time just doesn't see the router (5GHz only - 2.4GHz always OK) so they are strange things and as you say very frustrating, so maybe an issue with the EX6200 I use an ASUS RT68U router and thought maybe it was a compatibility problem between brands but its interesting that you have a nighthawk which should work and if you connection is good should not drop off.