PIX all of a sudden can't handle dns traffic

Brand new to PIX. Here's the story...
pix 515e running 6.3(5)
Box will freeze from time to time and won't pass/handle new dns
requests. IP traffic is still ok if you use ip# only. Host names will
not resolve. Clear local will fix the problem. I don't have any rules
or statics for dns.
Inside clients point to an internal solaris named box for resolution
who will pass request to isp if he doesn't have answer. Another fix to
the problem is if I change my pc's resolver from the solaris box to the
isp's dns.
I've been running a solaris based firewall for years in the same
environment and never had such a problem.(if I put the solaris firewall
back in place the problem goes away)
One minute I think there is something wrong with the pix because a
reboot or clear local will fix the problem and the next minute I wonder
if there could be something wrong with my named server because, as I
said, if I point to the isp's dns servers the problem goes away. No
changes have been made to the internal named server.
I can see from syslog that the dns udp teardowns that normally take 1
second all of a sudden take 2 minutes when the problem happens. I've
looked throught the log around the time that happens and see nothing
suspicious.
Any ideas - I'm going insane
thanks...jim

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