Is Xfinity Using a Blocker?

We have been having ongoing issues with our internet for the last four or so months. We pay for what is supposed to be a fast connection they offer for residential.  Our inhouse technology devices are up to date.  But lately, on wired desktops in the house and our wifi devices, we have noticed all sorts of error messages in getting to mainstream sites like Facebook, TCM, Network News sites, email providers, Ancestry, and Family Search, etc. and so on.  The errors are bad gateway warnings, time outs, failure to connect, etc. and so on.   Sometimes the warning comes up and the process simply stops.  Sometimes the "bad gateway" warning pops up, and the page loads.  We have called Xfinity, we have had our modem replaced with the newest model.  Is anyone else having this issue???  I almost feel as if Xfinity has a firewall program that is hosing up the connections, or not releasing the site parsing data.