what to do when clearing DNS cache fails?

dig {FQDN}

gives correct info from appropriate DNS server

ping {FQDN}

fails with wrong IP address, presumable cached from long ago. Flushing the cache is done differently on Lion (http://support.apple.com/kb/HT5343?viewlocale=en_US&locale=en_US):

sudo killall -HUP mDNSResponder

appears to succeed, but ping still fails with wrong IP address ...

{FQDN} is FQDN of Microsoft Active Directory domain, to which the Mac is bound, but DCs have changed & have different IP addresses now.

BTW - bug in Directory Utility on Mountain Lion - can`t get out of editing settings for directory without quitting entirely - is annoying.

Presumably unbinding the Mac from AD might help, but there doesn't seem to be an option to do this (and presumably will fail, since it can't contact the old DC?!)