Why does BT increase my noise margin every month or so

[#1]

I've been connected to Plusnet broadband since July 2006 and until Jan 2008 had a noise margin on the line of 6db and a sync speed between 3000 and 3300.
In Jan the speed dropped to about 1200 and I noticed the noise margin set to 15db.  Raised a fault and, eventually, got it rest to 6.
Month later it's back to 12 and the speed is down again to around 2000k.  Again got it reset, eventually, to 6db.
Now a couple of weeks ago I noticed it was back up to 9db but as this let me connect at 2880 which is fast enough to get a BRAS rate of 2500 I wasn't that fussed. 
This morning I get up to find the rate is down to 2300 and the noise margin back up to 12db.  Now I understand BT use the noise margin as a way to stop intermittent faults on a noisy line but until this morning my router had been connected for 7 days with no drops, disconnections, reboots and the line had been absolutely stable.
Anybody got any idea why this:
a) has started happening in the last 4 months
b) continues to happen even after getting it reset to 6
All MY equipment is happy to sync and work with a 6db margin and you can bet BT has better equipment than me.
Cheers
Richard

[#2]

Out of interest, what router hardware are you using? 
Depending on the model, you may be able to force a particular noise margin from your end, thus taking responsibility away from BT
B.

[#3]

The router I use is a Netgear DGFV338 which is not compatible with the DMT tool and doesn't even tell you the noise and attenuation parameters for the line.  I have to use an old DG834PN for that when I notice the sync speed is down.  The tools works with the old DG834PN but won't let you set it as low as 6 (well on my line) and doesn't survive a reboot.
Cheers
Richard

[#4]

yes i would surrgest using the same tool will work with most routers the only problem i have using it on a netgear dg834g v4 is if u turn the router off on resync the dslam uses its settings so i have to leave my router on all the time to keep my snr.
plus the dslam has default of 15db for my line its only possable using the tool to get it down to 9db at 10% setting.
how have u managed to get your default snr lowered so many times i have tried to get mine lowered as i use to have a router on end of an extension cable giving my 15db snr got wireless and has never droped the line since but plus net made me do all sorts to raise it as fault resulting in me just using the dmt tool instead.

[#5]

I just keep on raising the same fault with plusnet as my line is perfectly capable of running at 6db as it has been for the past 18 months.  Plusnet eventually manage to get BT to reset it and it's fine for a week/month etc the starts  heading up towards 15 again.  My router is permanently powered and connected but the DGFV338 has different firmware and is not compatible with DMT.
Cheers
Richard

[#6]

It looks like BTw possibly updated the DSLAM at your exchange in January 2008  with one that adversely reacts with the TI AR7 chipset in your DGFV338 router (large variations in apparent noise margin if I remember correctly). End result your target noise margin is raised to 'stabilise the line'. Is there a trigger event for when this happens - wet weather, etc?
There were a number of discussions in various forums about whether this was a real effect or just coincidence, see for example Kitz's forums here. I don't recall seeing a resolution to the conjecture, perhaps those affected bought new routers.

[#7]

The DGFV338 had lots of problems when I first got it with line drops so I've been using a DG834PN for the past couple of months.  Line dropped from 6 to 9 using that.  Netgear released new firmware for the DGFV338 a couple of weeks ago so I loaded that and have had a rock solid connection at 9db until 18:30 last night when the connection reset and the noise margin went up to 12.  This was 1 reconnection not the supposed 10 it requires to get BT to reset the noise margin.  We did have a sunny evening last night, that must be the unusual weather which triggered it 
The latest firmware for the 834PN is also better than the previous one.
I see from the Kitz forum that having interleaving enabled could contribute to this problem.  Wonder what it's set to on my line?
Cheers
Richard