Find the DNS server addresses in your router and change the DNS settings on one of the affected systems so that they specify those DNS servers for use by the local machine, instead of having your router handle all of the DNS queries. While you're at it... flush your DNS resolver Cache via the command line: ipconfig /flushdns
If that doesn't help, have a look at this thread
for a couple of very useful DNS tools from Steve Gibson over at GRC. Perhaps using some different DNS servers will also help.