Intermittent website loading issues when using Virgin DNS

I have an issue that is clear as day to me. All clients (wired and wireless) suffer an issue with random websites not loading intermittently when the DNS servers are derived from the router DHCP service.

See below tabs, I googled "result random" and opened the first 7 hits to a new tab. As you can see 4 failed to load

If I change to Google DNS (8.8.8.8 and 8.8.4.4), the issue completely goes away. The router has been rebooted again, to no avail. This issue first occurred 2 days after upgrading to 1GB download speeds from 512Mb (when I noticed performance issues and rebooted the router). Anyone have any clues?

An older issue with Reddit pages loading intermittently was also resolved by changing the DNS to Google DNS. Support have no idea how to troubleshoot this and closed my ticket because I didn't get back to them within 12 hours (was travelling long haul!)

Thanks