I have 1000Mbps from my ISP, by fiber. Archer C80 is my main router (receives internet straight from the fiber-to-lan conversion). My Archer connects via Ethernet (new cat6a cable) to a pc, that is capable of receiving 1000Mb of course. I can go days with everything fine, and then without any cause, internet speed drops to a cape of 100Mb. 

This happens totally randomly. I reboot, reconnect the cables, gets 1000Mb again, and after a few mintutes it drops back to 100Mb. More annoyingly, I can leave it as it is, come back in a day or two - and voila - we are back at 1000Mb, untill next time...