Hmmm, I wonder if something was "disturbed" whilst connecting your neighbour's new line.
I had that a few months ago. It took 3 months to fix a dodgy joint at the pole top DP, despite reporting immediate disconnections to the engineers up the pole on 2 occasions & also to my ISP.
The engineers just denied touching my connection.
It does seem too much of a drop for it to be just crosstalk, but SOMETHING seems to be hitting your SNR values.
If it's now 80/20 at the cab, it does suggest a problem between the cab & your house.
It really is a shame you don't have any "before" stats from when your connection was performing well