Vicious Circle

[#1]

My broadband speed seems to have got into a loop which I can't break. I'm on an ADSL2+ connection. Downstream attenuation is 44.5, which checks out with my distance from the exchange. All lines are clear, there are no faults as BT have done extensive work on my system. The router and only phone in the house are connected into the master socket through one of the ADSL faceplates. There are no extensions. This is what happens:
1. Router is reset automatically; download speed shown on the router (DSR) settles at about 6400; IP profile is 5500; download SNR margin around 6dB (which is what it is set at). Random tests with Speedtest.net are consistent at about 5300.
2. After a few days the router again resets. DSR goes up to about 6900; IP profile 6000; SNR hovers around 3dB; Speedtest.net results consistent at around 5800.
3. Another few days and router again resets. DSR goes up to around 7500; IP profile 6500; SNR drops to below 2.0, sometimes as low as 1.0; Speedest.net results vary randomly between 6200 and below 1000.
4. Shortly after this the whole system resets and goes back to stage 1.
Also the speed indicated by Plusnet doesn't usually have time to catch up and stays at 5.5.
My question is, would it make the speed etc. more consistent if the Downstream SNR was raised from 6 to 9? Or would this mess up the whole thing?
Incidentally when I was on ADSL1 on the ADSL2 connection, my IP profile was steady at 6.0 for several weeks, the current fluctuations only began after PN switched me to ADSL2+.
Any comments/suggestions gratefully received.

[#2]

Does this still happen if you remove the face plate and plug the router into the test socket (via a filter)? Have you tried a different router?

[#3]

The router is effectively plugged into the test socket via a filter, as that is what the ADSL faceplate filter does. Nothing else is connected to the master socket, i.e.no other extensions wired into it. I haven't tried another router but I have no reason to suspect mine. It is a Billion Bipac 7800N, fairly new, and has given me no trouble. Also, as I said everything worked perfectly before I was switched to ADSL2+.
I should add that I was previously on ADSL2+ and had a similar problem so PN put me back to ADSL1. It was after that BT sorted everything out on the line and so after some months of stability it was back to ADSL2+ hoping things were sorted. PN say they won't now switch me back to ADSL1 again as it is their policy to have everyone on the more advanced system where it is available.