Hi @Noisynoodle,
The staff here probably won't be around now until Monday, but they should be able to respond to this post early next week to help.
It may be that Wi-Fi Optimisation has switched back on again (if the router has been factory reset, or has applied a firmware upgrade), but if the 5GHz channel keeps changing, and the staff here confirm Wi-Fi optimisation is still switched off, then it is possible you are being affected by Dynamic Frequency Selection (DFS). All routers that broadcast a 5GHz signals have to comply with regulations that mean they check to see if the channel they are broadcasting on clashes with military / weather radar. If your router picks up a signal from military / weather radar on a 5GHz channel that could clash with the one your router is broadcasting on then your router must change channel. This is a legal requirement, and this functionality will be build into the router firmware, it can't be disabled.
Some more info on DFS can be found here : http://www.technical-direct.com/en/operating-wi-fi-network-5ghz-band-make-sure-deploy-dfs-first/
But the staff here should be able to check for you early next week, to see if Optimisation has become enabled again for some reason.
