Is there any noise on your phone calls? Dial 17070 and select option 2, there should be no noise between the announcements.
You noise margin would appear to show a problem with your connection or DLM intervening in an attempt to stabalise your line.
As your connection spped is less rthan half of your max attainable you need an engineer.
I presume you've tried connecting directly to the test socket to rule out internal wiring.
Not really as adjusting the noise margin is one of the steps DLM takes to stabalise the line. Whatever there is a line problem somewhere that needs and engineer to fix.