my collie grey did exactly the same,he would pick and choose the rabbits he wanted to chase, but he would always chase and catch ALL the big stuff,everytime,i put it down to either early entering on my part,or too much collie,what i mean is he was very clever,i kept him til he died cos he was handy to have around the kennel but he was that frustrating i felt like shooting him sometimes!! I wouldnt keep another one like it tho,so personally i would get rid and get somethng with a dash of bull and saluki, atb