If you are running a fast tor exit relay this fix is especially relevant for you:

Nick Mathewson:

o Major bugfixes (exit relays, DNS): - Fix an issue causing DNS to fail on high-bandwidth exit nodes, making them nearly unusable. Fixes bugs 21394 and 18580; bugfix on 0.1.2.2-alpha, which introduced eventdns. Thanks to Dhalgren for identifying and finding a workaround to this bug and to Moritz, Arthur Edelstein, and Roger for helping to track it down and analyze it.

Debian packages are available via deb.torproject.org now.

If you want to stay with stable releases have a look at the following resolv.conf options:

https://trac.torproject.org/projects/tor/wiki/doc/DnsResolver#Tuningeventdns...