Two Problems: DNS and intermittent connectivity

I am having two problems - one I came to ask about and another that I see others complaining about here when I search for answers for the first problem.

1) I have a site that I regularly visit - http://poznan-project.psnc.pl/ - that used to work just fine. However, over the past few weeks, I have not reliably been able to visit the site. I HAVE been able to visit a different site that I have been informed is on the same exact machine. During that couple of weeks I DID upgrade my computer from Mac OS 10.12 to 10.13 and then to 10.14. I suppose it could be my computer, but I'm not sure how. 

The error I get generally says ERR_SOCKET_NOT_CONNECTED for all broswers other than Safari (which I typically do not use). However, if I connect to a VPN, then I can access that same site from any browser I try. I tried to change the DNS server on my computer, but nothing solved this problem. Is there a DNS server setting on the X1 router itself? It doesn't feel like it because my phone gets to that site just fine, and it is using my same internet. If I switch from ethernet to wifi, nothing changes on the computer.  It seems it is my computer that is the problem, but why?

2) My family has been complaining about the internet for months. It is an intermittent problem where the internet just seems to go out for a second and then it takes a minute to reconnect. This messes up games and zoom calls and, seemingly, the Comcast Roku app. It does NOT seem to interrupt Netflix etc, but maybe they have a better caching system to avoid glitches.  Others here have claimed to not have problems on devices connected via ethernet, but my computer is on ethernet about 3 feet from the router, and it also has problems that seem to fall in this same category. Do I have a bum X1 router?  for what it is worth, I had a router go bad earlier this summer and had to replace it twice to get one to work. Is it possible this one is having hiccups?