I have a adsl line that comes over a long drop wire that is located at the best part of a thousand feet above sea level.
Consequently I get a large reduction of noise margin during the night hours.
It usually falls from 6dB to about 2 dB in the early hours. I have a Netgear DGN2200 which clings on to a connection for days at a time but at a sync of around 6.6Mb/s on a line of 47 dB attenuation. I also have a Belkin router which connects about 1 Mb/s higher but drops the connection every night.
In an attempt to improve things I "accidently" replaced the BT80 on the end of my drop wire with a BT80-RF3 filer. A new BT branded one. Imagine my surprise when the noise margin sank to about MINUS 0.2 dB every night from + 6dB.
Needless to say the original BT80 went back in place quick sharpish. The attenuation stayed at 47 dB by the way.
That will teach me to play about with things that don't concern me.