I had problems with my line too some yrs ago..... chking internal line was almost getting impossible...... so just bought a new CAT5 cable and re-did all the wiring frm splitter to main junction of the building...... since then its been wonders.....
I am 1.1km away frm xchg by road...... so overall must be arnd 2km of wire frm xchg....... getting 26-27db down SNR....... Attainable Rate down at 22.5Mbps..... on 8Mbps line....
Your almost same length...... so there is more room to improve...... must get somewhere arnd 15db....... so keep trying......
Keep an eye on stability/sessions...... not just the SNR numbers....... so if ur getting longer sessions than before...... means line is improving.......