Like many questions, the only true answer will be known if you try swapping provider. Of course no guarantee that who switch to will be better.

Upgrade to FTTC might help even if staying with TalkTalk.

Thanks MrSaffron.

I suppose what I was really asking, in terms of whether swapping provider would help, was whether the "bad gateways" that you suspect could be the cause of my issue are under the control of Talk Talk (where swapping provider might then help) or just a general part of the BT ADSL infrastructure (where swapping provider might not make any difference at all.)

As you appear to be on LLU (clue in the upload speeds) then nothing at all to do with BT ADSL infrastructure. How TalkTalk operate and split its millions of LLU ADSL2+ I don't fully know.

Even if it was on BT Wholesale infrastructure different providers can still have a big influence on performance and gateways.

Great - thanks for the explanation!

This occurs most often when the router has been off for a bit and is then turned back on, or if the router has not been used much (eg. after leaving it on overnight) and then we try to use it. However, there have even been a couple of occasions where it did suddenly drop in the middle of a normal day, though that seems less common.

Switch it on, and leave it switched on, 24/7 .... doing otherwise can sometimes cause odd profile issues (remote monitoring sees it as instability)

Have you tried a different router ? You may be seeing the early signs of 'illness' in your current one.

Thanks Zarjaz.

I had heard that throttling can occur if the router does not remain permanently online. However, I don't think throttling should be taking download speed from 9 Mbps down to 0.2 Mbps... that would seem to be going too far!

And nevertheless, I have this problem even when the router remains on overnight, and even whilst on during the day; in fact it happened this evening, and required 6 router restarts before getting a decent speed back.

As for the router itself...I have tried using 3 different routers, and the same thing happens with all of them; hence why I think it points to a problem somewhere outside of the house.

Is your line from a small exchange, what does Samknows say about it ?

It's a medium sized exchange I think... it serves 12,000 residences, and includes TalkTalk as an LLU operator.

Ah, OK, so not the notorious 'resold BTw' service you get in some small exchanges.

Upload speed would not be as high as reported if it was the bad old IPStream