Hard to say what is causing the drops. As you say, it could be a fault with the hub but don't assume that as it could be a fault on the line, or, indeed, something else. If a Plusnet person picks up your post they can run tests to give a better idea of what is happening but they have been somewhat thin on the ground here recently.
It may not show anything but I would be inclined to try reporting this as a a fault by going through the checks that Plusnet recommend and then running the online troubleshooter at https://www.plus.net/apps/kbdfaults
That doesn't cost anything - other than some of your time - and, if there is a fault, gets it queued for action. Please let us know how your get on.