Disappearing DNS entries

I notice a recurring problem and thought I'd report it. Yesterday, the DNS servers (the *.hsphere.cc ones and my own custom ones) stopped returning IP adresses for my service domain, and for one of my customers. This caused the page to fail to load in a web browser and when using nslookup to troubleshoot, the following message was returned.

*** ns1.servicedomain.com can't find www.domain.com: Non-existent domain

After the control panel went down and came back up, the DNS for these two domains were working again. This morning, I awoke to find an email from a different customer who is complaining that his site is not loading in his browser. Using nslookup, I verified that he has the same symptoms (admin, this is ticket # 9941). Does anyone know why a perfectly good DNS zone would just stop working all of a sudden? Is anyone else seeing this? Is there something I can do on my end to fix this? Thanks for your help.

Four hours open ticket and counting... please, someone look into this.