Update to Strange DNS Issue

I have some additional information that's making me think what I saw on
Saturday was just a coincidence. We have two DNS servers that were
unable to resolve external addresses beginning around 4:00 PM MST on
Saturday, but I found out that they both had just been rebooted. We use
some version of BIND on Solaris 9 and it was verified that named was
running, yet external names would not resolve for a few hours. External
names began to resolve in a few hours with no intervention on our part.

BIND gurus, any idea why this might have occurred? If named is running
and internal names resolve *and* there are no problems with Internet
connectivity, what might cause external lookups to timeout for a few
hours only to resolve spontaneously? It seems a little strange and I'm
sure there's some important information that I'm missing that would make
it all makes sense.