EX8000 - stability issues

Thanks for the reply, plemax . The previous screenshot is how the EX8000 appears when it gets into this unstable state. 

I did coincidentally do a factory reset today after my initial post in an attempt to eliminate some bogus configuration as the source of my issues. I've attached a new screenshot of the EX8000 after factory reset, before it gets into it's weird state. 

I'm not sure I understand your suggestion about disabling the 2.4GHz extended network and it's connection to the backhaul. I take it you want to force traffic only through 5GHz extended --> 5GHz router. Is that correct? I can think of two scenarios this helps validate -

1. Issues with 2.4GHz connections (either to router or devices) put the EX8000 in some state that breaks the 5GHz but don't break the 2.4GHz connections
2. Issues with 5GHz connection to router mean that the EX8000 falls back to using the 2.4GHz connection as a backhaul instead.  

Are these what you are thinking might be going on? I've had a look at the LEDs on the front when it's in this weird state. From the manual, it says the "throughput" LED would turn off if it were using 2.4GHz for the backhaul but I haven't obseved that.

I'll give it a shot (disable 2.4GHz entirely) and will see what happens. If this fixes the problem then theres some serious bug in the firmware that has it falling back to a slower band without giving any indication of it anywhere. 

EDIT - no idea why my images aren't working, annoying. Attached one directly.