Ah! I understand. That sounds like a fairly sophisticated application, as it has to send different emails to different list members on a regular basis while maintaining a database of who gets what. Not really an autoresponder, or simple list manager at all, but a specialized automated mailing list manager.
I have not seen such a thing, though you might try hotscripts.com - if someone has such a thing available, you are likely to find it there. Good Luck!