BIND 9 not able to resolve few domains

[Manjunath H N]

>     I have configured Primary & Sec DNS with BIND 9.2.* on Solaris. I
have found many times my DNS servers  ( Pri & Sec ) are not able to
resolve few Internet domains but these 
>domains are able to resolve thru dnsstuff.com. I have updated DNS cache
file, kill -HUP <named>, restarted named,  but still same problem
persist.
>    Any expert can help me to get root cause of it?

Are you getting a timed out error for the responses??  also check your
forwarders,