Intermittent broadband problem any ideas welcome.

[#1]

First I’m sorry for the ‘wall of text’ but I have a problem with my broadband and it seems very difficult to convince anyone there is a problem because it is intermittent.
So a bit of background, our telephone cables are a mixture of both copper and aluminium and are underground from the cabinet to the house, they are now 35+ years old and due to many changes, digging up etc are getting a little flaky.
Last year we had a similar problem in so far as the speed would drop and the downstream SNR would jump up from around 6db to over 11 db, this would cause a drop in downstream speed. It eventually got to the point where the speed would drop to 256K and noise would appear on the telephone line.
Our local BT engineer spent some time diagnosing the problem and after re-setting all the joints between the house and within the cabinet (including fitting a new NTE5 ADSL filtered faceplate and box), I think, did a ‘lift and shift’ at our local exchange (about 6 miles away) and this fixed the problem.
Since then we have had a rock solid 5Meg connection with around 6db SNR downstream.
Until last month……
At some point towards the end of last month we started getting speed changes so started monitoring the line stats again. As you can see by the graphs the problem occurs in the evening, usually after a warm day followed by a cold evening.
The router will suddenly loose the ADSL synch and re-set. The downstream speed will drop to around 3 Meg and immediately or shortly thereafter the downstream SNR will jump up from 6db to 11/12 db. This will stay like this overnight and then if I re-boot the router in the morning, return back to the 5Meg/6db and stay solid all day.
When this has happened before (and it has many times in the past), it is usually traced to the connections in the cabinet at the end of our road. The engineer has often said the cables there are flimsy and brittle (and I believe a bit of a tight fit) and the expansion/ contraction or movement when other repairs are done in the box causes problems (but this is only hearsay).
So what have I done,
Tried 2 routers of different make, both give the same results.
Tried a ‘quiet line test’, it is very quiet.
Reported it as a fault no 41074565, did the on-line fault checker, the team did some checks and asked BT to turn ON interleaving. Why I don’t know as interleaving has always been on on this line because of the distance from the exchange.
Tried to upload some graphs to ‘My Questions’ but got a message it didn’t like the filename (it didn’t say why or what was wrong) and refused to upload it.
Further tests it seems were done and because it was daytime passed with flying colours.
As far as the Plusnet team are concerned the fault is fixed, sorry no it’s not, but thanks for trying so far, intermittent problems are the pits to fix.
Please don’t ask me to try the ‘master socket’ I do have one but it’s just a 4 month old BT NTE5 ADSL splitter and I would rather not take it apart unless really necessary.
No I don’t have SKY, no nothing has changed at my end, no there are no extensions on the line, can’t think of anything else.
Last evening it re-set again so I’ve put it up here as a request for ideas or what can I do next.
Regards, P

[#2]

Hi Pendragon,
you need to return the fault ticket before it closes stating why you think there is still a fault.
Jojo

[#3]

Good morning, Joanne,
Done, but as before I was unable to upload any of the attached graphs, so I've put in a link to here for whoever to have a look at them.
Don't hold out much hope of getting a fix until the line either gets noisy or speed drop below 1Meg  ;).
Thanks for the reply, regards, P.

[#4]

Hope it gets sorted, it's definitely he best thing we can do for now.

[#5]

Do the file names of the graphs you are trying to attach to your ticket include spaces or non-alphanumeric characters? If so try replacing these by underscores ( _ ), and remove the special characters.