Intermittent DNS resolution problems

[maxxer]

Hi.
I've setup a new system with 18.1.12. We have 3 WANs, set up following the guide on the docs.  I've also enabled Unbuond DNS resolver to provider better DNS to the LAN.

Unfortunately sometimes the DNS is not responding, and so far I was unable to track down the problem and understand where it comes from.

As per the guide I've set up a DNS for every WAN (google and opendns), and if I go to Interfaces > Diagnostic > DNS lookup it always work fast (even for 127.0.0.1). But if from the PC where I perform the test via web I try a dig it returns timeout! I repeat the dig command for two or three times always returing timeout! Then, suddendly, I run dig once more and it returns immediately the result.

I've configured the firewall DNS rule (chapter 5 of the guide above), even if I don't exactly understand why it's needed: if the DNS IP is the firewall address, why should a LAN connection use the gateway?

Any hint on how to debug the problem  is very welcome. thanks