First, thanks Tido and ruir and sorry for not responding earlier -- I was in need of a working solution so eventually had to use a different device, but I would like to get back to this now. I did scan for the least-used channel and tried that, but I still needed to do enable_wmm=0 in order for it to become stable. I also watched the signal strength and without this it was unstable, so my feeling is that it's something to do with hostapd, however I couldn't get a build environment working (on Debian Jesssie) and I don't want to get into Ubuntu just to build Armbian (perhaps when I have more time).