Touch wood but since I opted out of BT Wi-fi the hub has behaved itself. It seemed to work better when I first opted out so I opted in again and the problem started up almost immediately. As a result of that experiment, I have permanently been out since June.
I had this exact issue so very reassuring to read this thread and thought I'd share how I got around this problem.
Firstly I was sent a Hub 6 following recontracting with BT, we unfortunately don't have fibre in our road s and subsequently every BT/Open Reach employee has said to stick with the Hub 5 which is apparently optimal for copper.
Secondly we did had two successive line faults which we incorrectly thought were the root cause of the poor broadband speed but weren't as the engineer was able to demonstrate 14Mb to the point of the line/connector into the Hub but then just terrible inconsistency on both the Hub 5 and 6 post the faults being fixed.
However the root cause for us is 100% the 2.4MHz v 5MHz issue but again complicated by Sky Q only using 2.4MHz.
(Also being a 100% Apple household, I believe Apple devices prefer 5Mhz so I solved the problem the following way).
Hub 5 connected to an "Apple Airport Time Capsule" wireless router (The Airport then becomes the transmitter of the 5MHz Wifi, (BTW the Time Capsule aspect is a 1TB hard drive that automatically backs up the iMac).
In the Admin settings for the Hub 5, I turned off JUST the 5Mhz Wifi. So the Hub 5 only transmits on 2.4 MHz
The Airport Time Capsule transmits the 5Mhz Wifi (it also optimises to the exact location of the Apple hardware rather than transmitting a traditional radius which again improves device performance
Sky Q, connects to the Hub5/2.4MHz Wifi automatically with no issues at all and removes the need for an ethernet cable
Additionally we have an Apple Airport Express wirlessly connected to the Airport which acts as a Wifi extender.
This has 100% solved the issue, we now get a consistent 11-12 Mb which is fine.
Too much time wasted trying to solve this issue, hence sharing my solution.
Thanks for telling us how you solved the problem. The time capsule was discontinued around 16 months ago, along with the rest of the Airport range but I guess that setting up any other router the same way will save the conflicts in the Homehub. I still find that allowing other people to use my internet connection through BT wifi & Fon triggers the problem and by not joining that, I don't suffer from drop outs.
I think the opting out worked as I didn’t really have a problem after that!
it’s not really been an issue after I switched to EE either