Hey Charlie,
We had the same problem here in Fl......the culprit was blown in
cellulose in the ceilings. We discovered this doing a rebuild after the
hurricanes a couple years ago. It explained why the house was so dusty
all the time.
I talked to an insulation guy I know, and he said thats a common
problem with cellulose once it starts to break down a little after
being up top for a while.

Take care
Steve