Recently had a similar problem, it was deemed a 'noise' issue by the telecom engineers. They came with what looked like a hand held AM radio and walked all around the house searching for noise peaks, nothing very special so they toured the court I live in and discovered that a neighbour, with a huge satellite aerial, had a set top box that was causing everyone in the locality to have major connection and speed problems.
Owner agreed to disconnect and stop using set top box, everything now just fine!!!