Orbi RBR50 / RBS50 bandwidth drop

Hoping someone can help me.  I'm 30 nanoseconds outside of Netgear's appaling 'you get 90 days then your on your own' paid support system so it's down to folks on here :)  

I have my Orbi router and two satellites connected to my broadband router using AP mode. Most devices are connected to the Orbi satellites by Wifi, but I also have a few wired up with Ethernet cables.  I have a 60mb BB package and have a PC connected to the BB router to check what bandwidth I'm actually getting and it tends to be around 55mb / 60mb which i'm happy with.  

For most of the time the Orbi system works well, and I'm getting between 25mb - 55mb to any device connected to it, and again, I'm happy with those numbers. My issue is that on a regular basis the Orbi system drops to around 500kb for no obvious reason.  It could be a few hours, or it could be a few days, but it will happen for sure.   Worth noting - devices never lose connection, they just get a stupidly low speed.   I know it's the Orbi system as I can test the actual bandwidth out of the BB router by checking on the PC that's connected to it.  It affects both of the satellites and all of the devices whether they are connected to the satellites via wifi or ethernet.  

Powering off all 3 of the ORbi units will usually fix the issue, sometimes it takes a few power cycles. 

I've tried a number the suggestions I've found in the community here including

- Changing the channels for both 2.4Ghz and 5Ghz.
- Enabling/Unenabling Beamforming and MIMO and WMM. Under Advanced 

- Enabling / disabling : armor, Circle, Daisy Chain, Fast Roaming, IPv6 and Set 20/40Mhz Coexistence to 40Mhz only. Set Short preamble instead of Long preamble modes.

Anyone got any ideas?