Here's my Friday data:

My Broadband Ping

See how I rebooted at 2am and then the sync was 500k or so higher, then at 8pm the disconnection forced it back down by about the same amount.

The ping changes are incidental as far as I know. Plusnet gateway random factor.

Getting back to the original post, I have a bipac billion 7700n and have exactly the same problem.
If left on overnight, the snr drops from 6 to 4, but only recovers slightly the following morning.
This causes a re-sync after two or three days. It looks like it could possibly be a router problem.

I switch the router off at bed time and re-boot in the morning giving me the best possible sync.

That sounds like the noise is wiping out some frequencies. Bit-swapping will initially increase the usage on others to compensate, but it comes to the point where that can't be done any more so the sync becomes unsustainable.

A drop of 3 or 4dB overnight is common. What is less common is for it not to recover at dawn. However, even when it does recover, as I understand it no reverse bit-swapping takes place. (Maybe it does with some kit, and that's why they hold sync better? I dunno - just wondering).

Thank you Roberto.
Problem solved. I have disabled bitswapping. The SNR drops by around 2 at night and now fully recovers next morning.
My line is now stable with practically no re-syncs.

Great .

First time I've heard of that, but I'll bear it in mind for others. Then see whether it is repeatable on other lines.