We had the same issues after upgrading from SH2 which worked fine before the SH3.
Our problem was that we have about a dozen things on our network which the SH3 can't handle. We would see devices randomly dropping off and when we got a list of devices on the SH3 page some of ours wouldn't be there!
We bought a RS600VPN and that worked for us - this may be a bit overkill but I wanted the problem sorted and I didn't have time to spend hours messing about. We also have an external TP Link wireless access point which is far better than the SH3 one.
We have most things on Ethernet so we have an external 24 port Gigabit switch too.
Our SH3 is in modem now with its WiFi disabled - completely stripped of it's privileges and left to do the only thing it can't mess up.
Just got the TP R600VPN switch working and everything seems to be working fine. That's all thanks to you and 'Monty Biggles' ... Much appreciation to both of you. I'm very new to all of this and have learned quite a lot ... mainly, to not take Virgin's next superhub edition!
So, I've just received my replacement SH 3.0 hub since I was having problems with the ethernet ports and after reinstalling and reprovisioning it I enjoyed 10 minutes of connectivity.
Now the ethernet ports don't work anymore: "not working" though is a misnomer because they all blink green and I can see some wired devices in the "Advanced Settings - DHCP - DHCP lease table" but NOT in the "Home - Ethernet connected devices" table?!?
I'm now going onto Amazon to purchase a router for the wired part!
VM please sort the firmware, I suspect the DHCPd part or fix whatever hardware is regularly failing!