I see two possiblities that you should investigate. If you have a good filter in the air conditioner and it is clean, then the dust is in the ducts leading from the a/c to the rooms. If the filter is dirty, replace it and this may eliminate much of the dust that was being sucked through the system. There are many companies that will come and clean your ducts, but this can be expensive (but also sometimes necessary). I think that if the new filter clears up the problem, then this may only be temporary until the filter clogs again. Keep a close eye on it.