when the internet connection drops, can you still logon to the configuration page for your router? of the top of my head i think it is
http://192.168.2.1/ for belkin routers.
If you can access this then try releasing and renewing the ip address that the router is getting. If you can't access it when the connection drops it suggests a problem with the router.
Also, when the connection drops, try a traceroute from one of the pcs to both fqdn and ip address (one ip for bbc for example is 212.58.224.121). How far does this get?