Fully in agreement with b*cat's suggestions.
Some years ago I suddenly started suffering an incredible rate of CRC errors, rendering the broadband almost unusable, but telephony was fine. As it happened (don't ask!) there were 'U' links before the master socket, and all was fine testing from there.
Changed the master socket and all was well. There had been a close strike about a week before, but the socket showed no sign of damage.
Before b*cat asks - sorry, I can't remember whether the old socket showed any abnormalities on a multimeter after removal.