I've read about this issue occurring for a few other members so I'd thought I share my yet-to-be-resolved issue.
I had a Cisco DPC3825 modem (in bridge mode, for what it's worth) which worked great for many years. About a week ago I started noticing slowdowns where my 100 Mbps connection would drop to about 0.5 Mbps. This was fixed by power cycling the modem. (I also had a tech reprovision the modem which seemed to work no better/worse than power cycling). It got worse and worse to the point where I had to power cycle the modem 5+ times daily.
Today I had the modem switched with one of the new Hitrons (not currently in bridge mode) at our Shaw store, only to discover the problem is still occurring...
I've seen in another post a Shaw rep suggesting that this is an issue with the Hitron that is to be patched in an upcoming firmware update, however in my case the issue occurred on the old Cisco as well - Seems to be something deeper than just the modem.
Happy to share any other experiences if it helps.