dnscachex rare abnormal behaviour

Dear all,
We run tinydns and dnscache in ISP environment. It has been running 
flawlessly for years. However, in recent month, two incidents happened, both 
of the same nature. First time, dnscache(x) started to return localhost 
address (127.0.0.1) when asked for google.com. Second time, an IP of a  
000webhost.com was  returned when asked for a very popular local social 
networking website. Both times, restarting the service solved the problem. 
We run djbdns version 1.05, built from source (Gentoo). I have checked list 
of root servers in dnscache(x) settings and it is identical to the one 
published at http://www.internic.net/zones/named.root. dnscache(x) is set 
to respond only to our client networks. No IPSEND is configured. We have 
default size of logs, so by the time we have noticed the problem, the 
interesting pieces of information were already gone.

Is this a sign of an attack or a bug? 
How can we avoid that in the future? 

Thank you very much in advance.

VK