I have a simple and typical network configuration consisting of an ISP-provided cable modem connected through a SonicWALL router/firewall to a single switch that has been operating flawlessly for years. Suddenly all users on the network are experiencing intermittent inability to connect to specific WAN URLs (e.g., server unavailable, no response to pings, etc.,) which change over time while others are accessible.
Often I'm simply unable to connect to a page (without any delay or struggle, the page simply isn't there):
This happens while I'm able to connect without difficulty to other URLs (with no delays or degradation in speed at all). In fact, it's common to be be able to connect to a page, but be unable to access the resources linked to that page on other domains
When this happens, I get "hostname can't be found" errors for the missing content, as if it simply didn't exist on the Web:
Again, this failure is by domain: some domains are reachable without a hitch, while others are simply "not there". It is never the case that I can reach some resources on a domain, and not others. Moreover, when a domain is not found, it remains that way for a several minutes: multiple attempts to reach it report the same "not found" error, until it "reappears" and is then accessible as usual.
My ISP has been to the site multiple times and run diagnostics, and has found no problems. They replaced the cablemodem, and I've replaced the router/firewall with a new one running default settings. But the problem persists. Oddly, it seems to be worse during certain periods: I can go for several hours without any issues, and then have a patch of an hour or more where various URLs become unreachable for 20 minutes at a time.
I have no clues in my router logs, which are generally quiet and show nothing that correlates with the occurrence of this behavior, and I have this problem when there is no traffic on my network.
What could be causing this and how might I address this? I'm at a loss since this is essentially an out-of-the box basic small-business/home network setup. Could my switch need replacing too? Is there something that could be happening on my ISPs end?