Next time You speak with telecom support, tell them to DECREASE the speed of the line. They will tell You that they are measuring, bla, bla,... that the link is OK for the X speed, bla, bla,... I struggled long time with unstable ADSL until one wise support lady did decreased the speed. Later, when problem arose (it was once or two times a year) I knew that someone upped the speed, so it was easy to convince support to decrease again.