Does ASA 8.4.3 check the source IP address of a DNS reply and drop it if the reply address is different to that in the query?
Customers DNS server does this due to a recent change, their server now has a virtual address, but replies are sent from its physcial address. This is temporary. Their PIX is happy with this.
Replace the PIX with the ASA, DNS fails, the only reason I can see is due to the way their internal DNS operates.