Chronic loose stool

Question
I am at a lost about my 8 year old German shepard.  I rescued her and the previous owner noticed when they first got her after about a month or 2 on a certain food she would develop loose stool. Before I got her(in March 09) she was pregnant and had loose stool throughout the pregnancy and I can't seem to clear it up now.  The vet has runs tests and can't find any abnormalities.  I started her on Science Diet I/D 5 days ago after fasting her for 24 hours.  She has been having liquid stool since and started today having blood in her stool.  I have been wondering if maybe a raw diet may be better but don't know where to start.  Any advice would be great as my vet is also at a lost.  Thank you

Answer
Could be an allergy or food sensitivity.  Science diet has a lot of ingredients, and she may be allergic to something, even to grains.  

To clear up diarrhea short-term, I fast a dog for 24 hours, then feed cooked white rice with a little chicken in it for 2-3 days.  She should also be getting probiotics, you can buy these at any drug store or health food store, such as acidophillus pills, it replenishes the good bacteria in the system.  I use a product called "Plant Enzymes and Probiotics" it helps digestion.

Raw diet is great, and I know people who claim it helped their dogs digestive issues, it's not too complicated, or you can buy frozen diets- here is an introductory article I wrote with links at the bottom: http://dynamicdogschicago.com/raw.html

Here are some of the probiotics I mentioned:

http://www.allisonsallnatural.com/servlet/the-753/Animal-Essentials-Plant-Enzyme...  (thats the one I use, its a powder you sprinkle on the food)

http://petenzymes.com/ (another good one)