Hi
Same thing happened here, at two different addresses in completely different cities, started off good and slowly got worse over time, managed to just maintain 80/20 in my current address before switching to FTTP.
The first address I was second on the cabinet, and SNR was about 13db I think from memory at 80/20, but in stages stepped down and I ended up sync'ing just under 80 before I moved house, the dslchecker also dropped as well putting max attainable at around 70 from 80.
My current address VDSL had been around for a number of years before I moved in so I was kind of hoping less of a change over time, started off at 11db and ended up around 6-7db after 3 years before I moved to FTTP. During an area wide power cut at my current address with me on a UPS and I guess everyone else thrown off line, SNR jumped to 15db, so cross talk was accounting for a 50% drop in margin.
I always say with xDSL is it only gets worse over time, until you can jump to a different technology and if that is xDSL as well you start the downward spiral again.
Regards
Phil