You need to keep on badgering BT!
It sounds like an underground joint (where two masses of wires of join up) has become split. The seal around the joint is made up of paper and a jelly like vaseline encased in a rubber 'cover'. If there is any ingress of water then it will affect the physical copper connections.
It won't affect your voice calls to much but will add to the resistivity of the data traffic - consequently the line looses strength and the data packets are affected. Not to sure why the problem would manifest itself regularly (ie. every two minutes) unless there is standing water 'lapping' around. That one is harder to explain!
However it could take them a long time to find the problem as they'll have to depend on lots of faults being logged which by analysis will show where the 'last point of pefect' connection was. They'll then be able to say - with some certainty - that it is the next joint which is the problem.
Nav