Line noise

[tehidyman]

I have been investigating why my download speed dropped from about 3Mbps to about 1.3Mbps between my last check on 25 Oct and next check on 7 Dec last year.
I have done numerous checks involving IDnet staff including running from test socket, alternative router, etc. etc. with no improvements.   I have turned off many of the electrical appliances in the house with no change in noise when turning off and sequentially back on.  I have wandered round the house with an AM radio tuned to 612 with no clues.  As part of my checking I have run RouterStats on my Netgear 834g v3 updated with latest firmware. My attenuation is stable at 50db. If I reboot the router it syncs at well over 3000kbps but within a few days has dropped to 2304kbps. BT tester shows download speed is 1859kbps, DSL down 2304 up 448 and IP Profile 2000.
The Noise margin seems to be the big variable, running steady for several hours at values between 7 and 12 db.  However at certain times of the day the margin drops dramatically to 3, 2 or 1db. This starts about 1600hr gets to its lowest at about 1830 but by 1845 has stabilised at 6db.
Reading the well-informed comments on IDnetters I have learned that this pattern is not unusual.  I was not running RouterStats when getting the higher download speeds so am left with the puzzle as to why the drop in November.  One change that has happened is that my exchange (Camborne, Cornwall) was upgraded to ADSL2+ in that period. Is it possible that the signal strength to ADSL Max was reduced then which would in turn affect my SNR ,margin and then reduced sync?  Another question is does signal strength drop as a result of contention.
Any thoughts appreciated.

[Lance]

Signal strengths have not changed and the router will obtain the best sync possible depending on the noise about at the time. Contention doesn't affect the line, but as more and more users get broadband there are issues developing where a large number of cables run together in the ducts.

Did you make any changes to the wiring, introduce sky boxes or alarms to the wiring or even turned your central heating back on for winter in November?

[tehidyman]

Thanks for that reassurance.  I have very thoroughly checked back that no wiring, new electrical equipment was installed in that period.  No Sky box. Our central heating is Night store and I turned them off when running RouterStats last with no effect and the severe line noise also occurs when they are off. They have been installed for a few years and I maintained steady 3.5+Mbps download all last winter although I was then with a LLU ISP (AOL).

[Rik]

You've done everything I would suggest with the exception of trying an alternative router and filter(s). If these don't cure the problem, you can ask for a BT visit, but it's an expensive risk, potentially £170.

[kinmel]

Try a 2700HGV for £9 delivered it will hang on to the sync below a 3dB SNR