I am having internet connection speed fluctuations.
I'm on a WIRED (cat 5e) connection through a beefy Linksys router (model EA7300), and every 20-30 minutes my speed goes from 200-300 Mbps (normal) to 2-10 Mbps for 20-30 minutes (using Ookla speedtest.net). All other devices (wired and wireless) on my home network are affected in the same way.
First, what model modem do you have? Have you tried bypassing the router and connecting direct to the modem with ethernet? Does it happen then? Remember you have to power cycle the modem each time you change what it is plugged into.