I came across this a lot back working as a mac support engineer. It used to be a common issue with older routers that too many DNS requests would kill the inbuilt DNS server, effectively preventing people from using their internet connection. This was typically isolated to someone running a torrent, with their torrent client doing reverse lookups to the hundreds of connections that occurred. This crashed the router's DNS service forcing a reboot.
Workaround was to set the DHCP server on the router to give out the ISPs (or google's) DNS servers.