Noise margin - not a complaint but I am puzzled

[#1]

My default noise margin is 6dB which is as good as it is going to get and after a day or so the margin drops to about 3dB during the day because of tones being dropped out - over night drops to about 2dB
Now something happened at  about 18.05 a couple of days ago.
The noise margin dropped 1dB over a 10 minute period (that is the log frequency) and now instead of varying between 2dB and 3dB it varies between 2dB and 0.6dB - see pic
I could initiate a resync but that might just drop the speed quite a bit.
Now the really odd part about this is that the error rate hasn't changed with the lower noise margin.
Could it be a router reporting error or has something changed on my line

[#2]

Monitoring shows that the FEC's are increasing quite rapidly but the CRC's aren't
Still holding on with a really low noise margin - 1.8dB at present

[#3]

What looks particularly strange there to me are the step changes in noise margin. Actually it all looks rather strange  with the long periods of unchanging value. I think I'd question either the stats themselves or what I'm guessing is a screen scraper app to collect and graph them.
I do remember years back I had a Speedtouch 585 which is probably a close relative to the standard PN supplied router and the FEC errors reported were astronomical.
I think it's easy to get obsessive with line stats - I know I did in ADSL days and when on cable I made a point of regularly checking the power levels. Now I'm on FTTC and there are none unless I unlock the modem which I decided not to do. It all seems to continue working wonderfully even though I don't look at any stats....
So I'd say if it keeps runnnning and you don't see a pile of CRCs don't worry - after all there is probably nothing you can do about it anyway assuming you've been through the business of isolating the ring wire in one way or another.

[#4]

The other step changes are when the computer is switched off over night and Routerstats stops monitoring

[#5]

I couldn't make much sense of the time scale but I understand now. I can't make any guess at what happened with the genuine step change.
Are the long periods of stable margin during the day genuine? If so it looks like you are suffering from evening noise pickup and that is usually caused by MW radio pickup on the ringwire - have you done anything about isolating it - iPlate, filtered faceplate, just disconnecting it? I hope you aren't running your router on a plugin extensio as that is bad news because then you can't isolate the ringwire in that.

[#6]

It is something external which has had a permanent effect
I suspect - but can't check without a resync which I am not going to do - that there was a burst of noise which wiped out a number of the higher frequency tones and they haven't re-established

[#7]

It's likely to resync of it's own accord sometime. I'm not sure if frequencies that get wiped out completely ever do re-establish. Your initial post suggests that they maybe don't. It's been a long time since I was on ADSL and I really don't remember the exact behaviour of my own lines. There used to be a nice utility called DMT http://dmt.mhilfe.de/ which gave a very good graphical view of all the bins and how many bits each could carry but it only worked with some routers. With some you could tweak the default noise margin but injudicious use could just mess things up. I used it once to speed up the abysmally slow DLM at the time when I'd suffered an increase in the default margin through a short term problem.