Your grass may have a chinch bug infestation. The Florida Extension has a very good fact sheet on St. Augustinegrass (http://edis.ifas.ufl.edu/LH010)
and it describes what seems to be your problem.
You might want to contact your local Extension office in Lake County, (352) 343-4101, http://lake.ifas.ufl.edu/. They will know more about this than me, since I am located in central Illinois.
I hope this is helpful to you.