ADSL Ling line - drops in SNR

I'm having problems with my ADSL circuit. In the last couple of months, it has become unstable and suffers from intermittent drops in SNR Margin. The drops can be as much as 10db and usually last from a few minutes to an hour or so (quite variable). I have a long line which usually syncs around 1.5-2Mb, download, 9-10db SNR and is quite stable. The drops in SNR are causing the link to renegotiate down to silly speeds like 800K.

Strange thing is that these drops only seem to occur during the day usually between about 8am and 10pm. I am absolutely certain that there is nothing in my house that is interfering. They can occur when the only thing switched on is my laptop and the router.

I'm using a Draytek 2830 (previously a 2820) from which I'm capturing the logs so I can show the graphs to BT Engineers. I am currently waiting for my third Openreach visit and my ISP Zen is asking for a 'lift and shift'. While the BT Openreach guys seem decent enough, they have never witnessed the SNR drop so seem unwilling to do anything.

Now I'm starting to wonder if the SNR drops may be being caused by someone else's telephone line i.e. only happening during the day/evening, no particular times, with telephone call-like lengths?

My ISP, Zen, has been very good at chasing BT and is definitely on my side but I'm getting a bit fed up as this has been going on for weeks!

Anyone seen this before or have any insight into the underlying cause?

Doesn't need to be a telephone, but anything creating RF noise in the AM radio band within range of anywhere the phone line travels.

Wonderful. It appears that BT have tried to stabilise my line by capping the speed at 1.2Mb.

Unfortunately the original issue is still there, frequent drop-offs in SNR. So now, not only have I got an unstable line, but it's also a slower unstable line.

My wonderful graphs showing the signal drop-off appear to fall on deaf ears with BT. If they can't see a problem during the 5 minutes they're in the house then, as far as they are concerned, no fault found.