Has anyone had a problem with dry itchy skin on a Labrador?

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My lab has always had dry skin. We got her at ten weeks old and she had terrible dandruff. We figured it was from poor diet as she came from animal control. We put her on good food, with no meat byproducts, she gets Natural Choice canned and dry food. But, whenever she sheds, it comes back. This year, with such a weird winter - very mild most of it - she has been shedding on and off since fall. She itches all the time. Some place she looks like she has hives, but that might be from chewing herself. No fleas, no mange. Does anyone have any ideas?