Some time ago I wrote an article about fixing a VPN connection issue over a Draytek router concerning a UDP flood defense setting. Its turns out that the very same setting can also cause an issue with DNS, the symptoms of which I will outline below. For the fix, head over to the old article above.
Essentially the symptoms are simple. You browse the web as normal and then all of a sudden DNS for new sites (not in local cache) stops resolving. A router reboot is the only way to fix the issue. Increasing the flood defense packet ceiling as mentioned in the article above stops the issue from occurring. I said it was simple!
As to why this issue had happened to me once more, despite having applied an earlier fix, I can only conclude that the ceiling I setup earlier, being tailored to the speed of my ADSL link at the time, should now be higher as I have around twice the bandwidth I used to. I doubled the value and now all is well.