Then there's a high resistance joint that the engineer hasn't discovered somewhere between you and the cabinet, which is semi-conducting. Perhaps next time he will use the TDR equipment he has (Mole, Hawk, Exfo, JDSU) to locate and remedy that line defect. Is there any audible noise on the line? When he replaced the dropwire, was it a flat pair / quad?
cheers, a
EDIT: JDSU!