Disconnects/Noise margin problems after dark

[#1]

Can you try to check exactly what goes where.
To be specific can you see where the wires come into the house and trace them to the first socket noting the cable colour etc. for identification.
The reason for saying this is that the incoming BT cable will, or should, only have two wires connected to the socket which should have a capacitor in it
Is it possible that there is another junction box hidden somewhere

[#2]

I will try. However both wires coming into the master socket come up out of the ground / wall inside the socket itself rather than having an obvious trail to follow. Nonetheless I'll do some sleuthing when I get home and see if I can turn anything up.
I think the previous owners had cable at some point - there is an old Nynex box on the outside of the house - so there are redundant cables running everywhere just to add to the confusion. I'll see what I can see when I get home later.

[#3]

Just to clear up one point, the modem cable you are using shouldn't be a problem as long as it is short and no longer than the original cable supplied with the modem router.
If I've understood right, what you thinks is your master socket looks like the LH Master Socket in the picture?
What you've described as "Bundle 1" with three wires is likely to be the main problem, you need to discover where it is going or coming from. It is not conventional colours for a BT cable.
Also remove the rear plate from the box and see if anything is connected to the A & B terminals at the rear.
As far as "Bundle 2" is concerned you need to disconnect the Orange/white & White/orange from terminals 3 & 4 and at all sockets in the house.
Find the BT cable outside and see where it is going. It may in fact go to one of the other sockets in the chain, or as Jim suggested there could be another (hidden) socket/connector somewhere. Check what wires you have on all other sockets.

[#4]

Correct, it looks basically like that from the outside.
However there is no backplate, just bare brick with the two cables I describe as "bundles" poking out. What are A and B terminals?

[#5]

The back plate is the bit with the Test socket mounted on it (the front plate is the removed bit). The back plate should be fixed to a surface box or a flush box. See para.3 further down the Master socket page.

[#6]

OK thanks, understood. I'll try removing that when I get home and see what I can see. As I mentioned before the test socket doesn't actually work so I'll be surprised if anything is wired to it.

[#7]

I removed the back plate and nothing is connected behind it.
I tried removing the white/orange and orange/white cables from terminals 3 and 4 and this again resulted in no broadband, it seems that it requires the orange/white connection to terminal 3 in order to work.
The cables coming into that socket are all underground and I can't say for sure where they originate, there are a couple of things wired up inside the garage which is approx 2 - 3 metres from that master socket that look something like

but I just cannot be sure if that is where the cabling originates.
I'm guessing I'll have to seek professional help with this one.

[#8]

I'm not surprised nothing is connected to the A & B terminals from what you've described.
The fact that disconnecting the orange pair of wires on 3 & 4 results in no broadband in addition to the above suggests there is quite a lot that is incorrectly wired up which is undoubtedly the cause of dropping connections and poor performance.
You said a couple of things are in the garage, can you be more explicit, well actually photos will be best. If any are Telephone (and not actually electrical) then it could be a BT (Block Terminal) No:1 but more likely a BT No:18 see http://www.britishtelephones.com/bterm.htm.
I think what we need at this point is some photos of all these wires/connections, starting with the Garage, then your Master socket and all extension sockets from the master up to the top floor, then we can provide some advice on the best way to proceed.
You can attach photos to posts by using the "Additional Options and Attachments..." (click on the +) which is visible below the reply box if you've used the "reply£ button just below the last post, or after you click on Preview if using the Quick Reply box. You then browse to where you saved the photos.
On the point of seeking "professional advice" - everything upto and including the back plate of the Master socket is BT OpenReach responsibility and the last thing you want is some "electrician or similar" tampering with it as most haven't a clue about correct telephone wiring and some have been known to use alarm or other incorrect standards of cable for wiring! As I said, post some photos and we'll advise the correct approach.

[#9]

Photos are up on Dropbox.
The first is the box closest to the master socket
https://dl.dropbox.com/u/6810859/P1000489.JPG
You can see just through the doorway the wall where the picture is hanging is the wall on which the master socket is mounted, so I would guess that the telephone wiring comes through here.
To the right of this is another, larger box:
https://dl.dropbox.com/u/6810859/P1000490.JPG
https://dl.dropbox.com/u/6810859/P1000491.JPG
Sorry these second pictures aren't so clear, it's quite hard to get to as it's behind a big pile of stuff in the garage.

[#10]

Apart from attaching future pictures to the post as explained in the PM, they take a lot longer to load when on external servers.
The first box looks like an electrical one, rather than phone. Can you see any wire going from the other box on the right to the one in the first photo?
The box on the right is a BT18 and we'd need a picture of inside it with the cover off. Can you see where the wires go, the one going down as well as the one going up?

[#11]

Sorry, will upload relevant pics here in future, I was using the external site as the sizes were too large for upload.
There does not appear to be any connection between the two boxes.
Attached pic of inside of the box on the right and an indication of where the wiring below goes. Upwards it is just into the roof space unfortunately.
Edit: to add pics of the master socket and its wiring.

[#12]

Can't really see where the downward cable from the BT18 goes as the picture is a bit too dark, but no matter the other picture is clear enough and the BT incoming cable is the right standard. Now the cable going upwards is old standard 4 wires (orange, blue, green & brown) and it doesn't go to that master socket, nor does "Bundle 1" (or 2) come from this BT18. So you need to try and work out where this upward cable goes, either to another socket or connector box somewhere. The Blue and Orange wires will be connected.

[#13]

It seems impossible to tell, it's up into the floorboards and from there it doesn't reappear until it reaches the phone points in the various rooms. Certainly there is no visible box, I have hunted all over the house trying to track where the cables are routed.

[#14]

I'm sure that photos of the connections on all the other sockets will reveal something.

[#15]

OK, here they are.
The first image is the other socket downstairs, close to the master socket and the BT box. Used for Sky and the house phone.
The second image is on the second floor and is actually currently unused.
The third image is in the study on the top floor where the modem/router is connected (as you can see I disconnected the ring wire, this one was easy )
Does this help shed any light on the situation?
Edit: to add study on top floor