Speed problems - help and advice please

[#1]

I have a  problem with my ADSL2+ broadband, which seems to be slightly different from other topics on this forum, so thought I'd open a new topic to see if anyone can offer any suggestions.
Let me say first of all I have tried all the usual checks/tests, and also several other things to try and resolve the problem. I was also on the DLM trial which did help a bit, but unfortunately because of the problems I was experiencing I couldn't make full use of the trial, and it may be that something has been set incorrectly as a result.
My bb will normally sync at 5000 and runs quite happily at this, sometimes for up to a week. The usual download SNR is about 6dB, sometimes over, and the router stats show a download throughput speed of around 5800 and an attainable rate of anyhting up to 7400, although more usually around the 6800 mark. (I've added some stats below). All well so far. The problem is that most evenings and pretty well all day at weekends the download SNR will drop, sometimes as low as 0.1dB (!) and there is no bb connection, sometimes for several minutes at a time. At the same time the attainable rate can drop as low as 5000. Fortunately the router usually stays in sync and the profile holds, but not always. The whole thing will crash, profile drops to 3000, and it can take a couple of days to get it back up. Also, using the telephone during this time causes the SNR to crash. (during the day using the phone sometimes causes the SNR to drop, but not fatally). Very occasionally I hear noise on the line, but unfortunately it's not continuous, and of course when I report the fault to PN the lines goes quite for a few days so they don't hear anything wrong!
It was suggested to me a couple of months ago that the problem was caused by some sort of connection problem whereby someone else's bb connection was interfering with my. PN picked up on this and BT's testing showed numerous dropouts over a week long test, but no-one came up with an answer. I was told that the results showed no fault on the line. At the time I was using an old router which dropped sync at regular intervals; since then I have purchased a new one, which does seem more able to keep sync.
Several things occur to me, on which advice or suggestions would be appreciated:
Could there still be a fault on the line, despite what BT say, and would it be worth vigorously pursuing this to get it fixed?
Would turning interleaving off make any difference?
Could it be a contention issue? If so. what could be done about it, if anything?
Is there any way of altering the SNR margin? I think during the DLM trials it was set at 6dB, and this can't now be changed, but if it were raised to, say 9dB, would this reduce the drops to such a fatal level as almost 0dB?
My downlaod attenuation is around 44, the upload about 25. Should these be more or less the same? From what I can tell from research 44 suggests a distance to the exchange of about 3Km; I actually live only 1Km away (in Bournemouth) and even allowing for telephone lines going via non-straight routes, I still think the high attenuation may indicate some fault somewhere.
Sorry if this is a bit long and drawn out. I never had a really stable bb line, and it has got worse (more noticeable?) since I switched to ADSL2+ in December. I can't fault PN, they have done all they can and I think they have run out of ideas as well. Any ideas? If anyone needs more information, data etc., I'd be happy to provide it.
Current router stats as follows:
Mode: ADSL_2plus
Traffic Type: ATM
Status: Up
Link Power State: L0

Downstream Upstream
Line Coding(Trellis): On On
SNR Margin (0.1 dB): 43 227
Attenuation (0.1 dB): 460 253
Output Power (0.1 dBm): 0 131
Attainable Rate (Kbps): 6296 1004

Path 0 Path 1
Downstream Upstream Downstream Upstream
Rate (Kbps): 5895 444 0 0

MSGc (# of bytes in overhead channel message): 59 10 0 0
B (# of bytes in Mux Data Frame): 184 63 0 0
M (# of Mux Data Frames in FEC Data Frame): 1 1 0 0
T (Mux Data Frames over sync bytes): 1 1 0 0
R (# of check bytes in FEC Data Frame): 14 0 0 0
S (ratio of FEC over PMD Data Frame length): 0.9987 4.5309 0.0 0.0
L (# of bits in PMD Data Frame): 1594 113 0 0
D (interleaver depth): 32 1 0 0
Delay (msec): 7.98 1.13 0.0 0.0
INP (DMT symbol): 1.12 0.0 0.0 0.0

Super Frames: 3753707 584380 0 0
Super Frame Errors: 0 0 0 0
RS Words: 243990972 0 0 0
RS Correctable Errors: 195925402 0 0 0
RS Uncorrectable Errors: 28549174 0 0 0

HEC Errors: 4294967289 2486386 0 0
OCD Errors: 5672 0 0 0
LCD Errors: 14 0 0 0
Total Cells: 819228030 57711 0 0
Data Cells: 6834943 85 0 0
Bit Errors: 0 0 0 0

Total ES: 51118 0
Total SES: 46558 0
Total UAS: 32 4294967268
Thanks in anticipation.
Alan

[#2]

Quote

Could there still be a fault on the line, despite what BT say, and would it be worth vigorously pursuing this to get it fixed?

Possible, but lets find out all normal avenues before we start getting BT involved (if the problem is found within your wiring/household, then a charge of £165+VAT(IIRC) can be charged..)

Quote

Would turning interleaving off make any difference?

Yes, it would. But the difference would be that the line speed increases, by becoming more unstable.

Quote

Could it be a contention issue? If so. what could be done about it, if anything?

Contention will not affect sync in any way. Contention just means your traffic goes slower because there is more coming from other people.

Quote

Is there any way of altering the SNR margin

Im not sure on this one, i know it can be reset to 6db, i think PN can request it be set to a diff level, however the DLM will change this at will(i don’t think it is fixed)

Quote

My downlaod attenuation is around 44, the upload about 25. Should these be more or less the same?

No, download attenuation is always much higher than upload. (e.g. Mine =
Line Attenuation (Up/Down) [dB]: 34.0 / 54.5
)

Quote

Also, using the telephone during this time causes the SNR to crash

If i am honest, it sounds like that you have a faulty microfilter/splitter. Do you have a microfilters on every point at which something connects to the phoneline? (Alarm, SKY box, telephone, Broadband router)
If yes, try swapping it for another.

also it is quite possible that the phoneline you have is that long.

[#3]

Matt, thanks for the prompt reply. To take each of your points in turn:
I appreciate that if the fault is in the house wiring BT will charge me. I am 99.9% certain the problem isn't in the house. In the past year  BT have rewired from the distribution pole to the house, and I now have a wire length of no more than 6 metres from the main connection outside the house to a NTE5 master socket, which is fitted with an ADSL v10 filter. One telephone is connected to this, and links to its partner wirelessly via the phone's own base unit. The only other phone we have is connected to an extension from the back of the baseplate into another room, about  10 metres in length, and fitted with a filter, which I have changed a couple of times in case that was the problem. The router is plugged straight into the v10. I did wonder if the v10 was faulty (it was new 12 months ago) and so have take n it off and plugged directly into the test socket with a 'normal' filter but that didn't make any difference. BT even changed the master socket baseplate at one time.
Your points about interleaving and contention noted. Obviously taking interleaving off isn't going to help if it makes the line even more unstable! With regard the DLM being locked, I have a feeling it is. When the DLM trial finished your colleague Dave (Townsend) gave us the option of leaving the DLM locked, and I'm pretty sure mine is. Further advice on that approach would be appreciated.
Phone causing the SNR to crash - I think I've covered your thoughts on that in the outline of my set-up above.. We don't have anything else attached to the phone system. When BT rewired we removed masses of old and redundant wire, so I know there isn't anything else.
BTW, I made another discovery today which may or may not be relevant. The bb has been quite flaky today; the SNR has tended to hover around 2.5dB even during the day, and a couple of times the bb has locked but the router has stayed in sync. Speedtest.net results were very slow. Normally to overcome this I would reboot, but I want to avoid that if possible in order to keep the profile up. What i found by chance is that if the phone is used, when the call is finished the SNR goes up slightly but the bb goes back to normal and Speedtest results go back up. Don't know if this helps diagnose the problem or just confuses the issue.
Thanks for the reassurance about attenuation.
Any other thoughts or suggestions for things to check would be much appreciated. It does help having someone to discuss the problem with!