Extra loop length and interfererers
Somebody needs to put a real time-domain reflectometer on the line to detect needless extra wire that might have been added at some time in the past (say, to reach a then-new location without running a new pair from the exchange). Also, line multiplexers (allows two subscriber circuits to function over a single pair) wreak havoc with DSL signals. Other interference sources like noisy lamp ballasts should be checked. Finally, any available alternate pair should be tested for better performance, and swapped in.