huge internet speed fluctuations on all devices including hardwired to modem.

With router disconnected and pc wired to cm1200 fluctuations were the same. However I believe I fixed the issue. I live in an old duplex where the old owner had done the wiring. Though I didn’t know this could effect anything I took your advice and removed a splitter I found in the cable line that had been used to provide cable to a different room in the house makeing one cable with no splitters to the modem. I have not have trouble since and the errors in logs seem to have stopped . Thanks for the help. Out of curiosity what makes this cause problems? I’m assuming it changes power and adds Noise to the line? What would be the appropriate way to run cable to separate rooms in the house if splitter cause issues? Thanks again for the help.