This is now fixed, with assistance from the mods, who organised an engineer visit. The engineer visited on time, and agreed that the fault lay outside my house. In fact, he was unable to synch his test kit when connected directly to the incoming pair. He tried changing to a different port on the card in the street cab, but that was no better so he put me on a port on another card. The modem then achieved synch in 30 seconds, and I have 36 Mbps downstream, with an IP profile of 37, and 5.6 Mbps upstream, with a profile of 10, which has been sustained for four days. Furthermore, there have been no disconnections during that period, so complete success. Once again, many thanks to the mods in general, and Stuart in particular.