I have what sounds like the same issues - since I've had my first superhub 3 (I'm now on my 3rd!) I get constant updating and periodic broadband access. This has been a month with literally no internet access - just what you want in the run up to Christmas. The superhub should update once to the latest firmware and that should be it. I've had all sort of nonsense explanations from the engineers but I suspect they dont have any idea whats causing it either.
Just to give some more details - we have had similar sounding probelms with each of the 3 superhub 3s that we've had installed. The units are frequently updating/restarting. The last engineer did line tests and confirmed that the power and signal readings were all within the acceptable levels which should rule out a physical connection issue - this was something that the previous engineer stated could be an issue. I dont see how the problem can be anything other than a network or superhub issue - and given that we have had 3 of them now I think its safe to say that unless they all suffer from the same fault that its not the superhub either??
I think the firmware is just badly outdated. I kept the hub updating until I woke up and then phoned technical support. They sent a signal again and then 20 mins later the greens are gone and the internet connection from my devices are good. I think it's fixed now, however it took 13 hours just to update.