My 5 yr old Dobe gets the same thing every spring, although she sheds what seems like 5 pounds of hair a day, she doesn't get bald spots, but the dandruff is terrible. The vet says she's in great health and it's nothing to worry about. Don't know if her being an indoor dog has anything to do with it.
Anyone test her thyroid level? That could be it any easily treated with a pill. Otherwise try omega fish oil supplement for dogs and a good shampoo like Allergroom or Chlorhexiderm or Hexadene. Good Luck!