Restart (USR2) isn't that reliable because of how ruby/rails needs to reload. You should use a full stop/start cycle instead. Once you start doing that and you still get no response then run strace on the mongrel process to see what it's doing. It's most likely that you have a library that's messing things up.
Also, don't use pstore, don't set Logger to rotate, don't do file locking, and don't use a crappy DNS.
- http://www.ruby-forum.com/topic/97043