My (then) ADSL line suffered in the same way as your's for many years, with DLM taking corrective action and increasing default SNRM etc. Eventually, purely by chance, I found that a very long electric fence 800m (and in the opposite direction to the exchange and cabinet) was the culprit. The large CRC errors didn't have any noticeable impact on response or throughput, but the DLM actions very much did so!
I did not know the fence owner, and did not feel happy about approaching him about the matter.
In due course, a Plusnet support chap helped enormously, and after several tries of different actions decided to have DLM turned off on my line; that proved very successful, and my line was then able to operate happily for 18 months, even with the CRCs present, until fibre arrived in the village.
The CRC errors were also initially seen on FTTC too, but within 48 hours were neutralised by interleaving and INP, and the problem and symptoms then totally vanished.