I believe there are multiple servers that actually serve up the site. And than you have a load balancer sitting in front of those servers. When you request a page the load balancer tosses the request to one of the servers at random to reduce load on the servers. So that is why it works fine one second and doesn’t the next. If one of the servers is down and you get tossed to the down server it will take a bit for the load balancer to give up and send the request to one of the other servers.
I think there used to be 8+ servers your request could go to. But now that number is down to two. So if one is down you are more likely to notice the problem now.