My first suspicion looking at the BQM graphs is that there is some kind of electrical interference impacting you line... it looks from the graphs that something starts around 4:30/5:00pm'ish and last for an hour or so.... If so you might expect the SNR on the line to be impacted.
Is there anything in your house (like central heating/air con) that starts up around then, if not perhaps something in your neighbour's or on the way to the cabinet. Or could it be something in the router that is starting to fail, do you have a backup or another router you could try for a while?
