That looks very much like a graph of someone who is using a Thompson speedtouch modem/router.
Is that what you are using?
Yes, well spotted! It is a 585v7.
It is a little early to tell, but I think that it has been much better since I installed the 126.96.36.199 firmware. With the previous 188.8.131.52 firmware, the router would block traffic from the Ethernet ports every few days, and had to be restarted. Sometimes I could restart the router using my mobile phone's wifi.