It may well have helped your line stability and your connection if when you connected to the test socket you had actually left it there instead of connecting back to master
if you have a look at some of my posts you will see i have exactly the same problem and ive had over twelve engineers out and had every test possible done including a tech wizard coming here and connecting up line monitoring equipment for weeks on end collecting info on my line and they captured this noise margin drops as yours does,the bad part is they tested for rein and theres nothing and the bt tech wizards could not explain the problem and fix it either so good luck with your problem.
Blimey I'm not holding out much hope after reading your thread.
Frustrating though when the noise margin drops along with the speed and then goes back up again when the noise margin increases.
I was hoping that it had gone away as I was synced at 57 - 58 mb since the middle of December then sync speed drops 12 - 13mb to 45mb just over 3 months later.
It is similar as my line speed drops when the noise margin drops to 3 or slightly lower and then speed increases when the noise margin goes as high as 9 before speed drops again at 3 and so on.
you need to increase font size for us old folk