1) If they're all connected wirelessly:
-Do you have any other wireless devices that could be interfering. Try setting your router to use a different wireless channel (preferably one several channels away from the one it's currently using).
-How strong is the signal? You may need to be closer to the router for it to work reliably.
-Try connecting one of the computers using an ethernet cable and seeing whether the problem persists.
2) You are currently using OpenDNS. Try setting your DNS settings to automatic, or try different DNS servers (Google, for example, have public DNS servers at 8.8.8.8 and 8.8.4.4). Instructions on how to do this:
http://www.mediacollege.com/computer/network/dns.html