DNS MX timeouts

[Vernon A. Fort]

I've run into a problem with named and timeouts primarily with MX 
lookups.  When a MX query fails the first time, i have to restart the 
named process before it will return a successful query.  Again, its 
mainly with MX lookups but it also happens with A records as well.  The 
problem subsides for 1-2 hours and starts happening again - basically i 
look in the mailq for deferred messages with MX lookup failures.

This box is a Gentoo install running a medium volume (500K per day) mail 
server - lots of dns queries due to rbl's, spamassassin, etc.  This 
problem started showing up around mid-may.  Since then, i have 
re-installed bind and bind-tools several times, updated the kernel, 
linux headers to 2.6.29, recompiled glibc, etc....

I just updated to 9.6.0-P1 from 9.4.3-P2 - same problem exists.  When 
doing a manual MX lookup (dig MX isc.org) - it takes around 45 seconds 
on the first attempt.  If it fails the first time, it will never return 
a positive query, just "connection timed out; no servers could be 
reached" until i restart named.  I can't say for sure but the bind 
application was updated around the time i noticed this problem.  All 
versions of bind i have tried (in gentoo portage) have the same problem.

Can anyone help me find where this problem might be?  I've google'd 
until my eyes are red and throbbing.

Thanks

Vernon