If you graph is accurate, that's a very orderly series of drops.... could it be electrical noise causing this ?
What kind of thing would cause the electrical noise? There's been no changes in the house recently, certainly nothing near the router. Or could it be something along the line?
7 minutes at a time, every 30 minutes. Is that pattern repeated 24/7, only in the evenings, only on weekdays, etc., etc.?
Heating system springs to mind.
It might be worth an evening of switching off (at the wall) everything electrical you possibly can - heating, lighting, the lot - and see if the pattern continues.
After that, I'd follow the route of the cable and see if you can see anything nearby that could be generating a lot of electrical noise.
Externally, you anywhere near trains, level crossings, pumping stations? Any factories (especially fabricators - all that welding is noisy)?