We have proved your line will connect at 8128kbps. That will give you an IP Profile of 7150kbps after a few days, then depending on BT you will get 6500kbps or more actual at good times.
What we have to do is get the same performance from the outer socket.
What might be worth trying is to replace it, still without the phone, and again get the full stats immediately. Purely a confirmation test that there is a problem with the wiring to that removable faceplate, as opposed to a one-off incident as mentioned by MHC.
Sorry to get long-winded in what follows, but we are trying to get you up to those high speeds permanently through the master, and once they are installed any of the extensions as well.
What I'd like really is two or three photos, as I can't quite follow your description and some of it seems odd. Also, the terminals on the faceplate are numbered. What is/are the colour(s) of the wire on each terminal, and what number is that terminal? Which one disappears off into the rest of the house, and what's this about a box in the roof? Daft though it will seem to you, the colours and numbers are very important.
Are you saying the cable from the pole goes underground to the house, and if so does it come in through the wall to the master, or go into the roof to that internal box, and then down to the master rather than as you described?