I advise running the test socket experiment next.
A point on the comment about crosstalk...
No idea who told you crosstalk was ruled out, but unless they have a magic crystal ball I've no idea how they could possibly think to know crosstalk wasn't the reason.
Crosstalk can start on the line at any time.
The engineers may only connect new lines during the working day but a new line won't cause crosstalk and reduce your sync until that customer connects their modem. They can do this at anytime, midnight if circumstances required it.
A single engineer can't know every line that's activated on a cabinet, so if it was an engineer they are talking nonsense.
Former FTTC customers can be left jumpered to the cabinet and their line reactivated remotely. An engineer wouldn't know this has happened.
I've never seen an FTTC line that hasn't lost any speed to crosstalk, unless at the 40/55/80Mb cap for their package. Even then crosstalk has likely nibbled away at their attainable.
Some don't notice it as the lower SNRM targets can recover some sync.
I used to get 55Mb.
Crosstalk has dropped this to about 37Mb.
3dB SNRM increased it back to 45Mb.
You are on an ECI cabinet with no lower SNRM targets available to you so crosstalk would be noticed immediately and not covered up by DLM lowering the noise margin.
Edited by j0hn83 (Tue 22-Jan-19 11:38:18)