Problem solved. The AT&T technician changed the pair arriving to my house
with some unused ones (this is the second time) and now the impedance of TIP
& RING is virtually the same and not changing. No interruption has occurred
in 24 hours! The conclusion of the story is that the impedance of the line
should be the same for the positive and the negative, if it is not the case
the common mode rejection is greatly degraded making the line susceptible to
external disturbances.
Thank you again to you all,
Gene