ADSL (BT Phone Line) broadband cuts off intermittently for the whole network. "Sync" or "DSL" light on the modem turns off/flickers.
Possible connection problem on the line. Possible interferece / faulty equipment.
Check that ADSL Microfilters have been attached to all pieces of telephony equipment (Phones, phone base stations, burglar alarms, SKY set top boxes etc.) if it plugs into the phone line it needs a filter.
Unplug all telephony equipment from the line then plug one filter into the line and attach the modem. If it works with no problems this indicates that something inside the property is causing the problem. Start plugging devices back in until the problem reoccurs.
If it doesn't then swap the filter with a different one and try again. If it works with the second filter then the first filter is faulty. If it still doesn't work then contact your ISP (Internet Service Provider) who should be able to arrange an engineer's visit.