A bad problem in my DNS !

Hi
I have a problem in my DNS servers. I have two DNS. They have valid IP and have Zones. But they didn?t forward to any DNS servers. They work truly for hours but suddenly at minutes they can?t resolve a few domain names! Of course any time the DNS find the IP address but can not connect to the host. In this situation we trace that host with TRACERT command and in one point after a few hops the host is unreachable. What do you think? If I set IP of other DNS in outside of our network for forwarding, will my problem solved? or I must make a cache only DNS for my networks? At end my clients connected to these DNS servers. In advance, thanks for your response.

Mehrdad Khadempour