Thank you so much for your generous help! Once again. Much appreciated and good to hear from you again. I can thrash a modem as you say.
I’m even beginning to wonder if this is a bad modem. In the ‘hollow curve disease’ threads elsewhere the thinking is that my current modems have been damaged by a surge in jan/feb 2020. It was then that the long sequence of many faults began in all my four modems it now seems, because in every case we have discovered that power-cycling a modem cures the hollow curve disease. Neither I (stupid) no A&A could work out that this wasn’t a DSL line problem. Anyway, I’m wondering how many other problems there are in some or all of the current modems. I have a large stock of spares and just need to recruit my wife to help me swap the current ones out. That might mean that the status: idle thing would go away. It could be as rare as 1:1000. There have been other weird things going on with the modems. When I telnet into them sometimes, not often, telnet responds incredibly slowly, taking 20 s to come up with each CLI prompt for example. What seems to have been happening is that an Openreach engineer comes out, somehow reboots a modem - not sure how - and then can’t find a fault in the line, plugs everything back in, fires it up and the hollow curve disease is gone. It took what 20 months for the penny to drop, and both I and AA were pretty much equally dense. AA I think trusted me too much to do the usual have you turned it off and on customer service thing; they respected me a bit too much.
There’s no sign of any problems on the line when the status: idle thing has happened. I can check that with the clueless.aa.net.uk server’s line monitoring graphs and they show nothing funny going on, no loss of sync. A resync takes me about 70 s, so I would be able to see that clearly on AA’s line monitoring graphs.
You’ve clearly proven that it is not a general modem bug.
So the thing for me to do is to swap the modems out, which I promised AA I would do before xmas but still haven’t managed to get it done. I’ll makes sure that I also swap out the PSU bricks in case current dI/dt delivery is a problem. Saw a prototype machine at work once where slew rate wasn’t high enough and the machine needed a local capacitor to supply on-demand fast delivery current to satisfy demand spikes rather than trying to pull fast current through a line from the PSU that had unwanted inductance.