Eventually the webgui will not show the wifi config page properly, it only displays half the screen...
That piece just caught my attention... Are you using https to access the router? If yes, try doing a forced refresh (shift-f5) of the page. For some reason, the self-signed certificate generated by the router sometimes have issues with browsers (Firefox is the most problematic, but I have it appear from time to time with Chrome as well). I suspect it's the browser trying to establish a path of trust, and since the router's web daemon is single-threaded, it sometimes gets stuck, requiring a page reload.
I generate the certificate differently from Asus (to resolve various issues like the lack of SANs, which are now required by modern browsers as the CN field is being deprecated).
One fix that definitely works is using a certificate with a complete trust path (i.e. generating and installing your own CA in your browser). You can do that manually using a tool like XCA (what I use), or a simpler aemi-automated method with pixelsrv.