That all confirms there is a problem. Follow my previous suggestion (different router direct in test socket with no phones connected) and see how it goes. If it is still that unstable then it is almost certainly a line fault.

I think the LCP down is it losing sync with the exchange.

In reply to a post by Philnoob:

1) We don't have any filters installed in any extension sockets anywhere in the property. We were told the filters were not necessary because the router is plugged directly into the ADSL socket of the BT face-plate on the Master Socket. We were told that doing this precluded the need for filters and also meant the router was connected to a point where it could not be affected by problems arising from the house's telephone extensions

That depends on whether the extensions are filtered at the master faceplate. Faceplate may be faulty.

Worth a try putting filters on all phones incl any at master.

I once had a filtered faceplate fitted by BT which caused shushing noise on its phone. Put in filter on phone side and noise went away. Have since binned filtered faceplate.

Have you got a Sky box?

Yup: we have a Sky box on the middle floor. Again, I thought that couldn't disrupt anything because the extension it uses is 'beyond' the ground floor master socket / BT face plate.

Oh, a friend suggested I "improve" the type of wire that connects ther router to the ADLS face plate socket -- the wire we have is the thin cable that came with the router. But I don't know what type of cable would be required to replace that.

It was this friend who a year or two back disconnected the bell wire inside the master socket and yes, that definitely improved things for several months after.

Seeing as today has been disrupted with non-computing things I'll work through the suggestions and advice given tomorrow.

For now, sincere thanks again to everyone for all their help. I'll post back with an update in due course.

Some Sky boxes can require double filtering.

Assuming the one in the master socket faceplate is still working, adding a dangly filter at the Sky box extension may help. If the one in the master faceplate has failed - as has been suggested, it is vital to add one for the Sky box and everything else.

The stats taken using the test socket, while you are messing about, would be useful.

Thanks Roberto. I'll go dig out a filter that I know we've kept in a drawer somewhere and install it in the Sky socket. Sadly, I don't think we have any other filters as we dumped them after the faceplate installation. Tomorrow I'll post the stats from the test socket only connection.

unplug the Sky box as a first step in diagnosis.

You may need to try a different router, as your line is at least 63.5 dB attentuation and your service is somewhat borderline. The 2-wire BT Business routers do reasonably well on long lines, and some older netgear or zyxel's with TI AR7 chipsets were the only thing that would connect to extreme lines.

The oscillation between speeds and retrains points to a challenge in the router's ability to negotiate a successful setup with the exchange and sometimes another router type is the best cure.