Could be one of the filters is spotty(happens) Swap a known good filter costs about 5 to 6 pounds Have you done a line check by listening in? ( See My tip in the current copy of WU) or here and check the cable runs for damage? Then if all is looking right and no improvement then Ring BT for a line check
-------------------- History so far.. 8 years surfing & 27 Years Computing (First ZX81 1k)