DNS server stops responding to queries

[Andre_KT_ZA]

Good day

We have a server at a client that is the file, AD, DHCP and DNS server. Every so often it stops being able to resolve DNS queries for clients on the network. Even running nslookup on the server and pointing it to an external DNS server does not resolve queries. I suspect port exhaustion is the issue however I do not see much of a difference in the quantity of ports opened by the DNS.exe process when it works and when it doesn't (5024 and 5030) respectively. 

Anyone have any advice on how to proceed?

[#2]

The server could be running out of resources or loosing it's network connection. 

-Is there a backup running that could be taking all of the memory or CPU during this time? 

-Is it happening the same time everyday or at different times? 

-When it does happen are you able to remote to the server and all of its shares?

-Is the server service running when it happens? 

-Could there be not enough IP address's on the DHCP side?@Andre_KT_ZA 

-When you are having the problem is there a Time delay,  or is the time on the Server different from the clients?

-What is the reservation time set to for the DNS leases?

thats all I have right now off the top of my head.  Let me know if any of these fix your issue.