buffering on pc hardwired

I have used Xfinity for years at my other home in the SLC area, but at this new home the service has given me issues.   I moved to the other side of the SLC valley, and apparently, they just put in Xfinity wiring in the neighborhood, according to neighbors.   I have had the internet hooked up here for almost exactly one month and speeds are great.  Speeds test downstairs at the modem at a full 2 gig down and about 240mb up.  The wifi is good on the next floor up at about 600mb and then goes down as expected on the top floor at about 300 mb.   I get rare buffering on the second and third floor when streaming on wifi, but that is to be expected.  The real issue is with the direct connection to my PC via new CAT cable the installer provided with the new modem.  I am getting great speed as I mentioned, but I am getting lots of buffering on many websites and while gaming.  I have done the tests to check my connections via the Xfinity AI thing and it always says everything is fine.  I have done lots of reboots of both the PC and the modem.  I also get random errors about DNS resolution issues.    What can I test / try to fix this?