Hi
I had a similar thing, for 2 weeks over the summer holidays on ADSL2+ it would suddenly improve, then drop again exactly 2 weeks later, happened 3 years on the trot. Was it crosstalk or did someone have some noisy electrical equipment that went off when they went away, was it a factory taking a 2 week shutdown over the summer, no way of knowing.
If I was constantly monitoring the stats then no doubt I would be seeing little peaks and troughs constantly. People do turn their stuff off and on and noise comes and go, hence why there is a margin. You will even see changes in noise due to certain weather conditions and how it affects radio waves.
I think you just have to accept that the way the technology is, xDSL is highly variable and it is just constantly shifting sands, especially long lines such as yours that is already optimised to the maximum. I would just stop looking constantly, you'll drive yourself mad
Hopefully at some point you will get FTTP or even could take a satellite service, and the issues of noise and cross-talk will become a distant memory.
Regards
Phil