I agree about finding a holistic vet, there are a lot of herbal remedies a regular vet is not going to know about and all of them are much safer than steroids (like predisone) which have a lot of side effects and long term health problems associated with them. Until you can find out the cause, these might help relieve the itching
Try pure colloidal (colloidal just means ground fine) oatmeal. It's important to leave it on at least 10 minutes and not use hot water. Here's a link.
http://www.acnetohealth.com/general2.html
Also here's another link about other itch remedies - Have your tried 100% Aloe Vera?
http://www.homestead.com/bulldogsworld/ItchySkin.html