I have/had the exact same problem with my two Agouti berkshire ratties: Nugget and Sweet Pea. They always run away when I try and grab them, and once and a while they bite. I got really sick and tired of them biting me, so what I did was put some baby food or yogurt on a couple of my fingers. That way, when I went to grab them, and assosiated my fingers as something nice. They really haven't bitten me much since, just a couple of bites now and then. They still try to run, I haven't conqured that yet.