I would say a normal SNR fluctuation would be no more than 3db max any higher could indicate a problem with interference.
Are you on ADSL Max? What router do you use?
If you think you know the source of the problem e.g. a streetlamp you can use a MW radio to try and listen to find the interference, not sure of frequency, just tune around and see if you can pick anything up.
I use a tool which identifies the frequency of interference on my line (amongst other uses) but this is only compatible with certain routers. Here is a screenshot of a line (not mine) showing the frequencies of gaps at the bottom right click here If your router is compatible you could try it.
Have you removed the ring wire from your master socket if as this can act as a AM radio antenna?