Its not unheard of for SNR Margin to drop in the evenings - in fact thats when it usually does... hence me asking the question earlier on in the thread.
Various things can introduce Noise on the line - common known factors are:
Street Lighting, Xmas tree lights, Storms, Electrical Interferance.
Adsl traffic is also at its heaviest in the evenings and this extra traffic in itself can produce extra noise or crosstalk.
Sometimes the decline may be gradual, (say as traffic gets busier) or sometimes it will be abrupt.
Abrupt decreased are normally from an electrical source.
A typical example of this happening is on line 2 at the bottom of this page here
http://www.kitz.co.uk/adsl/mymax.htmSee the green on the graph - sometimes the SNRMargin is at 10dB, then it will drop right down to practically zero or on occasion below zero.
Please bear in mind those graphs are taken from maxdsl accounts therefore it can tolerate lower SNRMargins. On traditional adsl - which you have - drop outs can start to occur at below 10dB. But the graphs still show what can happen to your SNRMargin - and exactly why a margin has to be there in the first place.
PS.
Dont let that start you thinking that maxdsl could be a cure in your case - because being on max causes SNR to drop further.
See the graph on the left thats on an "excellent line". The drop you can just see from 31dB to 9dB was when it was switched over to max.