ACD's aggression toward cats


Barked: Tue Aug 17, '10 2:38pm PST 
I recently adopted a 5-7 year old ACD, and I adore him. However, I was ignorant of his breed's traits when I adopted him and the shelter assured me he'd been "cat tested" and got along exceptionally well with cats. He now studies my cat for hours, follows him everywhere, steps in front of him, and has softly growled at the cat. After researching the breed, I am very concerned, but he is showing complete submission to me. Any advice about how to help him understand the cat is not prey but a family member? I want to keep this dog at all costs, excepting the cat's life. I'm grateful for any suggestions.