Noise Margin of over 21 Billion db isn't good right? (Speed Issues)

[#1]

Had intermittent speed problems over the last week or so... connection speed can nose dive for anything from 4mins to 4+hours & sporadically pick up again for a moment or two.  Problems seem to occur 'mainly' on an evening & weekend... yeah I know, peak times etc etc, but it shouldn't (has never in the past) drop from 10+ Mb to 1 Mb.  Sometimes it's so bad I can't even load a page to complete a speed test, current record is 18kbps downstream speed.
Noise levels on the Router aren't encouraging.  Attached .pdf of BT Speed Tester results & router stats at the time of speed test.  Speed tests completed over ethernet connection from only phone socket with anything plugged into it.  Don't appear to have access to a master socket - house is only 3 yrs old & has a BT Openreach box outside which doesn't contain anything that looks like a master socket.  Also, took the faceplate off the first socket, no master socket.
As results are intermittent, I don't fancy getting stuck for a BT call out charge if they test the line when things are fine - anyone got any thoughts?
Thanks...

[#2]

You will need to do a bit of investigating.
Try following the cable from the box outside you may well find that the master socket is in the loft or the garage (if attached to the house)
The enormous noise margin just means that it went negative

[#3]

Unfortunately the external box is an 'External NTE' at the bottom of this page http://www.kitz.co.uk/adsl/btsockets.htm
As the page says...."no internal test socket for the customer"
Any other thoughts?

[#4]

As I don't know the answer and as it raises a worrying point I have posted a generic question here for greater visibility http://community.plus.net/forum/index.php/topic,90920

[#5]

Thanks for your help Oldjim
Hopefully someone from PN will catch this thread & may have some additional pointers/tests I can try, as these external boxes aren't that new
Most frustrating part is that the fault is intermittent & generally happens when I need it the least, i.e. working from home, project deadlines etc.... it's a shame, as I haven't had any problems with my PN service in over 5 years!

[#6]

The basic problem is that with a reported noise margin flickering about zero the error rates leap up to the point where the connection is unusable
One thing which possibly can be done is to increase the default noise margin - there is a trial where it gets locked but I don't know if that is still running.
I don't know if this is possible on ADSL2+ but it might need the the line retraining.
Of course one solution is to go back to ADSL1 which would solve the problem but limit your speed to 8128kbps
However before doing that you should really try a different router as that may just be the problem