I persuaded a tech in the shop to sell me some BT wire. Ran it from point of entry to telephone socket. A child could do it. Along with above got my speed back up from 2.6 to 5.2.
Old wiring can be faulty although tests show otherwise.
BTW your rate will drop every time the connection is cut a couple of times close together. And then return to the rate it was before - if there are no further cut offs in the next 3 or 4 days.