As the itchy spots are limited to the the areas where the straps and waistband are in contact with the skin, your symptoms could be due to contact dermatitis, that is exposure to an irritant like a chemical or metal or anything you are allergic to, it can cause rash. It could also be eczema. This itches so much that all the scratching makes it look like 'shoe leather' (or 'elephant hide'). You can control the symptoms with prescription strength mild cortisone creams. For either of these conditions for therapy the best way is to prevent attacks by identify those substances which you are allergic to and avoid them. So, Allergy testing would be worth looking into.
Hope this helped and do keep us posted.