Get online support
I have noticed that the download speeds are very inconsistent even during the day. I'm on the 100mbps plan.
I reset the router and managed to get fantastic speed even via WiFi, then after an hour or so it was back to the typical range of 40 to 60mbps.
Not a complaint, just odd.
Not sure if this is related to my drop-out issue question I asked earlier.
Can you take a screenshot of your modem sync stats when it is running slow, that will be helpful.
And say what it is, wifi or ethernet when you notice the slow down.