Interesting problem.
So, I would first set your DNS servers delivered by DHCP (in the router) to:
8.8.8.8
8.8.4.4
and see whether you get the same issues with the clients.
Also, ensure the clients are set to use DHCP for the IP address *&* the DNS servers, and as Spurlash says you should ensure the files:
c: \windows\system32\drivers\etc\hosts
c: \windows\system32\drivers\etc\lmhosts
either don't exist, or if they do exist (and you use them) you've cleared them of any spurious entries.
Also, if that doesn't solve things so far, then check whether you have any proxy servers set in your browser settings.
Finally, download Malwarebytes Anti-Malware, update it and do a scan to see if it can find anything infected.
Come back to us with the results, and we'll work on it further if that's not sorted things for you so far.
Cheers,
Mike.