If the extension was run from the correct place, then it will be connected to the terminals on the back of the removable bottom section of the master socket.
Oxidisation can occur on the tiny spring prongs that connect the test socket to the bottom section, as there is no movement between them.
It would be worth unscrewing the bottom section and sliding it in and out a few times, or better still, use a contact cleaner fluid. It may stop the disconnections, and also restore the speed.
My plan is to move back to the BT socket on the stairs when I get a plug socket fitted on there. But I will certainly try out the information you provided to see if this will rectify the issue. It will most likely be tomorrow when I try this out, I think it will be better to buy some contact fluid cleaner as you stated and give it a test.
Servisol Super 10 is a good contact cleaner, its also very good at removing sticky labels as well.
There may be other suitable stuff, but you have to make sure that its safe on plastics.
The actual contact area is quite small, and consists of a number of springy strips that make contact with the faceplate so you have to be careful not to damage them, or bend them out of shape.
You may find evidence of corrosion if the socket is situated in a damp place.
The wires to that extension should be visible on the back of the faceplate, as shown here.
It may be worth checking that they are connected properly, and there is no corrosion.
It only takes a slightly bad connection to cause random disconnections and loss of speed.