Interesting and puzzling problem

[#1]

I have been monitoring my internet connection for some time, using Routerstats, as the line has been a bit dodgy.
This evening I noticed that the error rate jumped up significantly during a phone call. I therefore decided to try double filtering by adding a rat tail filter to the phone output of my ADSLNation filtered faceplate. To my surprise while doing it Routerstats picked up a very large error spike and the router resynched with a lower upstream speed 384kbps at 7dB - fortunately the downstream speed didn't change.
This is a picture showing the effects - does anyone have any idea as to the cause

[#2]

Have you got a Krone tool Jim? I had a similar problem a while back, and after checking everything else to no avail, began to suspect the master socket might have a dodgy connection somewhere.
I cleaned the gold plated contacts on the faceplate and socket with isopropyl alcohol (meths would do too) and a cotton bud, and remade all the connections, including the incoming A & B wires (they'll never know  ;)).
No problems since. 

[#3]

There aren't any wires connected to the master socket except the incoming ones from BT as I disconnected the only extension because it was connected using bell wire.
I could check the A & B connections because, as you say, they would never know.
I don't have any isopropyl alcohol (not sure about meths) but I could use double distilled plum brandy (it is so strong it refluxes on the side of the glass) or CPU heat sink cleaner

[#4]

The latter sounds better, Jim - sure, the brandy's got plenty of alcohol, but a fair proportion of sticky stuff too I'd think.

[#5]

Seems a waste of good Plum Brandy!
A drop of Nail Varnish remover on a cotton Bud or even a drop of Vodka will do if you don't have any Meths.

[#6]

Removed the faceplate and checked the connections which looked OK but disconnected and cleaned them (I had a little bit of meths left - obviously I wasn't too thirsty last time  ;D) and refitted them.
The other thing was I made sure that the insulation wasn't trapped under the plate (it might have been before) to ensure a better connection.
Reconnected with this result - the speed won't hold overnight but it shows what the connection is capable of.
Quiet line test seemed a little better and the upstream sync/noise margin is where it should be.

[#7]

An update.
What I didn't mention was that in reconnecting the wires I touched them together.
This of course bounced a current down the line which must have cleared a bad joint.
It went bad again a couple of days later upstream 384kbps at 7dB until there was a call this morning which wasn't picked up very quickly so it rang for a while and the line resynced with 448kbps upstream noise margin 19dB.
But the thing I really don't understand is why it only affects the upstream frequencies

[#8]

me definitely no expert, but dont forget that the upstream are a lot lower frequency that the down