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, our external DNS server resolves it just fine. 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.