Wlan connection reverting to manual is happening to me too, on used N9 bought a couple of days ago (initially I thought a damaged wlan antenna would cause disconnections, then I saw this thread).

Anyway at the moment this is my configuration:
- PR1.3 (I think stock one)
- no sim card, so wlan disconnection stops internet connection
- I'm connecting to hidden wlan: maybe I should try to let router visible
- it seems N9 gets lower signal on hidden wlan than other devices do: after loosing connection I search for available wlans and the hidden one comes up after a bit, showing lower signal