Intermittent name resolution issues

Hello, we have an internal DNS server (9.2.1) that seems to forget how
resolve different names on the internet at varoius times. For example,
it'll be happily humming along and then it won't resolve www.yahoo.com
however, other queries to internet hosts on the internal DNS server
resolve OK and our external DNS server resolves www.yahoo.com OK. This
behavior will continue until we either restart named or interestingly
enough, I've done a $ dig +trace -x <yahoo.coms ip address> <ip of
internal dns server>, and what's that has been resolved, it seems the
internal name servers cache magically knows how to resolve www.yahoo.com
again. Any hints, suggestions, clues. Thanks.