I have my ps4 connected to the lan cable and have a 100k line.
Test with internet connection will then also attracts me 90mbits, but after 1-2 hours it goes down to 10-20mbits at once.
Then I always have to take out the LAN cable and put it back in, so that it stays constant for a few minutes.
Does jmd know what it is? With another Lan cable I have already tried it but no change.
Go to the side of your gateway and see how much Mbit get there. Any speed tests bring nothing.
And see if anyone else is still connected. Maybe someone else is downloading something or you have an uninvited guest on the network.
With Fritz box stands 94mbits and in the WLAN are only which from our house (3 persons) and if times nobody is at home it comes nevertheless to the problem with the speed and the lags in the play.
Uninvited guest's also still I've just looked at the page.