This is an indication that your ADSL modem is unable to hold a steady DSL signal. This could mean that the modem is connected without proper filtering to the phone line, another telephony device is interfering with the modem’s signal, or there is a fault on the line.
Please check if there are any unfiltered devices attached to any wall sockets such as a phone, fax machine, Foxtel, alarm system, etc. and ensure that the ADSL modem is correctly connected to the wall socket, and that the line is not filtered.
If everything is correctly connected, try isolating your ADSL modem. This is done by disconnecting all telephony devices that connect to a wall socket, except for your ADSL modem which will connect directly to a wall socket with any filters.