We suffered irregular breaks in connection to the internet since joining the network last autumn. The breaks were generally less than a minute in length and often came in groups a half to an hour apart, though sometimes minutes and sometimes hours apart. Also the connection might remain unbroken for a period of a few days, though never more than six. They bore no relation to our activity on the internet or to congestion, as the breaks were as likely to happen overnight as at any other time. VOIP was also affected, with silences lasting a few seconds, but not necessarily associated with a notification of a connection fault.
Various causes were suspected: initially our router, and our ISP (A&A) kindly lent us a new one FOC; then some power spike from domestic equipment affecting the ONT - so we bought a radio and tuned it to 612khz; then a poor connection in the network. Finally the ISP called in Openreach, who checked everything, declared the fault likely to be elsewhere but as a precaution installed a new ONT.
That was two weeks ago. We have had no problems since.