Help with an aggressive dog after eating?

we have a 6 mth old Border Collie, Alfie, and a week ago brought home a 7 mth old Collie, Mollie, all was going great until last night. After we fed them (separately) both came into the living room and suddernly Mollie turned and attacked Alfie, aggressive showing her teeth. He is the most placid easy going dog ever, he turned his head away in fear. We put Mollie into the kitchen and kept them seperate that night, Alfie was cowering the rest of the night, next morn all is well and back to playing and running mad. Mollie is the 'boss' out of the 2 which Alfie accepts. Then again tonight Mollie was fed, I left Mollie in the kitchen for a while, when she came into the living room she immediately went for Alfie again. unprovoked, 10min later all ok! Mollie has already had a home and the older dog didn't take to her, we don't want to have to find another home for her, she is fabulous dog except for this but maybe she needs to be in a home with no other dogs. Please help :o)