If your router has multiple ports, then try the other ports... sometimes a router can have a flaky port.
If not, then do you have another computer you can try to see if the issue is related to the computer itself?
You can also try running multiple traceroutes from the computer to see if there’s something screwy with your DNS... perhaps one of your DNS providers is incorrect, resulting in some packets not making it out of there properly.
I wonder if the user has malware running? Hey poster, Have you run an updated copy of malwarebytes? Spyware will cause the problem you are having....
I should have said they are all wireless.