Next update: sorry to have to say this, but I'm not sure the ticket staff are actually reading what I am telling them. I keep getting responses along the lines of, "Your issue appears to be resolved. As your connection to our service is stable and your router is communicating with the exchange at the expected rate."
Well, it might be OK at the moment they look at it, but it only does that for a day or two before the problem reoccurs. I have even provided a Routerstats log showing big noise margin drops every couple of days, after which the router can't recover a decent speed without the awkward manual intervention referred to above.
