When it happens, it happens simultaneously to any PC connected to the router.
I still have wifi connection and ADSL connection when DNS is lost
Having wifi connection and ADSL connection doesn't mean that you still have an internet connection! Your wireless network my be working fine, and the ADSL lights may be working, but if your router has lost IP connectivity to its upstream device, you don't have an internet connection.
When it happens again, ping 4.2.2.1 - if it works then your internet connection is OK. If not, it's a connection problem, and nothing to do with DNS settings.
A few other things:
Do your PCs have their DNS servers set manually or via DHCP?
Your PCs may have their DNS servers set directly to the ISP DNS servers, a third party DNS (e.g. OpenDNS) or to the router address (it then acts as a DNS forwarder).
Bear in mind that what you set on your PCs - manually or via DHCP - is independent of the DNS server information that your router receives (assuming DHCP) from your ISP.
So if you are setting DNS explicitly on your PCs to any DNS server upstream of your router
and you still have internet connectivity, then DNS will still work, even if your router loses its own DNS settings.
My suggestion, to eliminate some variables, would be to switch DHCP for your local LAN off on the router. Set your PCs IP configuration manually, including DNS servers. The DNS servers you set on your PCs should be the ISP's DNS or an external internet DNS server.
Now it doesn't matter what the router does, your PCs are going to connect to a DNS server as long as you have a functioning internet connection.
SD