It is fairly normally for all lines to fluctuate during the evening. This is when SNR is most likely to be at its highest due to more people being at home, more electrical stuff being switched on.. or even just crosstalk on the lines since more lines are likely to be in use.
If you are on a long line, then unfortunately it is a common feature that if you sync up in the early part of the day at a high speed, then come the evening time then the line will drop and have to re-negotiate. :/
Each time a line is dropped and re-negotiated at a lower speed, then this is recorded by the
DLM / RAMBO In cases such as this, then the constant renegotiation of sync speed can cause the DLM process to think that perhaps you need a higher target SNR. Once the higher target SNR profile has been applied then that is something that is much harder to get rid of and will affect the maximum speed at which you can connect in the future.
As an example Im on a very short line - but even my SNR margin fluctuates depending on whats going on around.
Monitoring the SNR as you say is perhaps the best way to see what is going on, and perhaps help us see if the amount of decrease is acceptable. Its probably best doing this not only in the day but also the evening.
In some cases it is best to leave the router switched on and let it find its own level. Although the router has its own power supply, processor etc.. the one thing that it cant remember across a reboot is the best level for the line which it was previously sync'd at.
