Slow connection with large SNRM variations

[#1]

My connection is slowly reducing in speed and reliability, after dropping from 2Mbps at about the time the nearby FTTC cabinet came online!! I now get between 256kbps and 1.1Mbps with (relatively) frequent disconnects. I know my line isn't good, but a couple of years ago I had 3.2Mbps with no re-trains for months (before 21CN arrived at the exchange!).
Current basics (Rx/Tx)
Attn: 59.5/36.6
SNRM: 9.5/6.3
Sync: 760k/740k
A couple of SNRM graphs attached: this AM from a Thomson 585 (which then decided it needed to re-initialise the firmware settings) and this PM from a Netgear DGN1000, both sampled @ 10sec intervals. The Netgear produced similar variations on Friday 20th July.
I cannot identify any internal equipment with starts/stops etc which match the sudden drops/recoveries.
The connection is via 10m cabling (all IDC joints; CW1308 & CAT5e) from a filtered faceplate. When I get permission from higher authority, I will run a mains extension so that the router can be connected directly to a dongle filter in the test socket and linked via wireless! However, I don't expect much change as the last time I did that the attn/snrm/sync figures didn't change significantly, if at all.
Any comments on connections to the test socket - I will have one corded phone plugged into the dongle filter and the RJ11 cable supplied with the router.
Phil

[#2]

If your wiring is up to scratch, then you are unlikely to notice any difference plugged into the test socket. In that case you would be very unlikely to see any changes in attenuation, what you may see is changes in SNRM and sync speed, simple because you had a resync and line conditions may be different from the last sync. The reason that you should plug into the test socket (and you should try another modem/router to ensure your normal one hasn't gone faulty) is for your own protection. If an engineer is dispatched and the fault is occurring but stops when he connects his test gear and you aren't in the test socket he can turn round and say it must be your wiring etc. Then you may get a £60 bill! Anyway I see you've tried two modem/routers and if you plug into the test socket with dongle filter that should confirm no problems with your filtered faceplate (or otherwise!).
Those SNRM graphs are horrible, it's likely there is a bad joint somewhere having been disturbed as a result of the upgrade work. First check is to see if you have a line problem - can you hear any crackling or other noises on the line when using the phone? Do you have any problems with outgoing or incoming calls? If yes, confirm with a corded phone direct into the test socket. This then has to be reported to your line rental provider (don't mention the broadband). If it's Plusnet life will be much easier.

[#3]

Anotherone,
I did have crackling on the line a few weeks back, but Plusnet got that sorted and a "quiet line test" is now fine - also no problems making/receiving calls before/after. However, there has been no noticeable change on the broadband side.
I shall move the DGN1000 later this evening and leave routerstats running to see what, if any, differences there are. That will cover what's needed for the troubleshooter