DNS Problem on 3910

I have an odd problem.  

I specify the DNS servers I want to use in the 3910 and historically I use Quad9. 

After a couple of months I noticed that the network started running slowly, I never tracked it down, but a bit of 3rd party software also started playing up about the same time so after performing some diagnosis I noticed that the QUAD9 DNS servers were not resolving the DNS queries.  Suspecting the QUAD9 server had flagged the site as suspect, I switch my DNS severs to Cloudflare and everything started working again and the main network perked up again so thought nothing of it

However, roll on a couple of months and I have the same problem.  Network slowing down and the same bit of 3rd party software not working again.  And sure enough an NSlookup to a domain is no longer getting resolved. But it is Cloudflare and if there was a DNS error on cloudflare the whole world would know about it. 
 
Which leave something happening in the 3910.  Anyone have any ideas?  The 3910 is running 4.4.5.2.

Haven't had the problem bar ipv6 falling into a confused mess when I was using dual wan. Eventfully it would stop working and everything would fall back to ipv4 only.
I'm no longer on dual wan so no idea if that's been fixed or not. I've noticed that you're running old firmware so first stop is to update that. I'm on 4.4.6.2/v28. Back up everything before hand just in case it goes tits up of course.
DNS is using the servers I've sent under custom. 
 

Good point about the firmware....

I have two of these (contingency) so will upgrade the backup one over the weekend and switch it over to production.

Not confident it will fix the issue, but I should have thought of this, DOH...