urgent problem with nameservers

[chilihost]

I have an urgent problem with nameservers. I have a number of cpanel servers running their own nameservers (each one has 2 dedicated nameservers running on the same server). This has been working perfectly for the last 6-12 months. All of the sudden this weekend, I have started seeing a problem where I can no longer resolve domain names to IPs on these servers. It started out last thursday with one server and now it is upto 5 different servers behaving exactly the same. There were no changes made to any of these servers in the last week.

Here is what is happening, which I find rather strange:
when I ping a site from multiple locations around the world, it does not resolve
when I dig a site on multiple nameservers around the world, I get this error:
global options: printcmd
connection timed out; no servers could be reached
however, when I dig a site on its own nameservers, I get a proper response with IP address.

It does not seem to have anything to do with the nameserver's registrars, since I can see this exact same problem using both enom and godaddy registered nameservers. It also is not affecting every server that I own, or at least not yet. Other servers are running with registered nameservers on the same domain name too, for example one server runs with nameservers ns1 and ns2.server1.mydns.com and it does not work, while a second server runs with ns1 and ns1.server2.mydns.com and it is still working.

Can anyone help me solve this mystery?

thank you!

[chilihost]

one other thing, when testing these domains out on dnsstuff.com or dnsreport.com, they resolve properly and provide the correct IP address responses.