Frequent disconnections, looks like an UPSTREAM problem though

[#1]

Hi folks, this is an odd one.
Firstly, it's not my problem, but it is a PlusNet customer that I was helping this afternoon with something else and decided to have a look at his long-running broadband problem as he's not at all familiar with how it works.
If a staff member wants to PM me I can tell them more technical details and the account name so they can hopefully help progress towards a solution, unfortunately I don't have a ticket number - couldn't get online long enough to check!
He's using a Netgear DG834Gv4 router so I decided to have a look with DMT. This showed a bizarre tone graph.
Downstream was pretty normal, considering he's now on a15dB target SNRM, but upstream was very odd.
The upstream graph was low and pretty flat, with typically only 5 bits per tone. Upstream speed was only around 300kbps though I did see it once hit the magic 448.
Resyncs were frequent - typically every couple of minutes so uptime was minimal compared to uptime. This goes on all the time, not just a specific time of day.
Any thoughts anyone?

[#2]

In my case it was a faulty line card but took 3 BT visits to sort it

[#3]

He thinks BT have already changed the card at the exchange, but I guess they may have just swapped the tie pair or even actually done nothing....

[#4]

It might be worth trying another router as there was some evidence that my Speedtouch 585 was making the problem worse

[#5]

Beat you to it, some time ago in fact......

[#6]

... and the filter?

[#7]

Tried several of different makes.

[#8]

Hi HP,
Drop me a PM with the details please and I'll take a look. Does sound strange.