I think that there are at least to possible things going on here.
First, if the name is resolvable from elsewhere (and I'm fairly sure this is not the problem given your drop in hits) and not from your own client, then your local DNS resolver has been compromised.
If the name resolves badly from everywhere, then your DNS authoritative host server has been compromised.
The good news is that those problems are easily fixable and by now should have been fixed by you (if you support your own authoritative host and resolving DNS servers) or by your ISP if they paid any attention at all to the huge story last summer about this topic.
FWIW, the authoritative DNS and the resolver DNS servers are using the same type of software (DNS), but they are doing different functions. The authoritative server is there to serve "answers" to requests about your domain (from its host file) and the other serves to ask "questions" about domains from other DNS authoritative servers. Any good administrator will setup their DNS servers to allow them to only resolve the names they serve (as authoritative) and only resolve names for clients in its known network. Sadly there are a huge number that act as what is called an open resolver, meaning that as an authoritative server, it was also answer for other domain names (though it should not do so authoritiatively) and as a resolver will serve anyone out there. Both are probably unwise and certainly not recommended configurations these days without a darn good reason to do so.
If this is the case of an ISP who did not "get it", then they are reaping the harvest of paying no attention to a critical infrastructure problem. The bug relates to randomness of UDP ports for a DNS server. Patched versions of every major ISP have been out since mid-to later last year (in fact it was the biggest coordinated infrastructure applications fix in the history of the Internet).
Don't feel bad, your ISP is not alone. The stats on current DNS servers which actually address the bug are dismal. To me, if you want to be in the Internet business, you should know that something as basic as this kind of attack must be addressed if there is an answer.
While something else more nafarious could be going on, this would be the most logical guess and probably the starting point I would check first.
-Commerce