Check out the following bug report. I would also look at other bind bug reports. My sense is that redhat has deviated quite a bite from the ISC version of bind. In particular I believe that they disabled or otherwise modified the caching behavior back about 6-8 months ago when there were major security issues with bind. I have felt that my Red Hat/Centos name servers have not worked as well as Fedora or ISC bind name servers since this time. You might try installing ISC bind and see if that solves your problem.
Nataraj
Interesting - though in our case it's failing long before a few million lookups. I don't much relish compiling ISC versions to run on my box - the security implications and other hassles don't seem trivial. [We don't allow external [the world] lookups - just local "trusted" users, but that only mitigates some of the security concerns.]
Perhaps it's possible to use an older version that's security patched. Ugh.
-Greg