I checked the driver date in the other PC and it was the same.
The troublesome PC has completed it feature upgrade of Windows 10.
I monitored the performance of the DSL-3782 connected to the master socket and it has dropped the broadband connection after about 16 hours and reconnected at about 8/25Mb. Such behaviour not acceptable but known to be caused by a bridge tap fault which can't be repaired. (Wiring built into house.)
I've reinstated the configuration with VDSL modem (in test socket) and HG533 (via PLAs), which gives good performance on all devices, and tolerable performance on the spare PC when it isn't exhibiting the problem of bringing the whole network down, which it isn't at the moment. I'll leave things alone for a few days now. It's good to know that there is someone to refer to to investigate this sort of thing.
I have no way of proving if this will work, as I don't have a separate modem, but what about replacing the HG533 with the 3782. Then connect the pla back to the modem to LAN1 on the 3782?
I am not employed by TalkTalk, I'm just a customer. If my post has fixed the issue, please mark it as the Best Answer.
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I might try that. I've just reboxed the 3782 and put it back in the drawer!
Back in the day, TT provided an HG532 with modified firmware so that the connection from the modem could go into one of the LAN ports, but I got my Netgear router which has a dedicated input port for the connection from the modem (but doesn't support IGMP, so the TV service doesn't work). The HG533 has a dedicated input port for the modem connection, and is configurable for use of either ADSL or VDSL. (It has a built in ADSL modem.)
I thought about putting the 3782 at the location where the HG533 is, and using the phone wiring to connect, but then I'll have the degraded service caused by bridge tap problem. So many options!
Thanks for the help so far.