My macbook (late 2008) is turning of by itself, what does that mean ?

My macbook unibody aluminum is acting weird, it keeps on turning of by itself. I use it withought connected to a wall and i can use it for around 12 minutes then it turns of by itself. So the only way i can use it is if I have it plugged in. I want to know why it does this and what I have to do next ? The battery life is at 89% and the cycles are at 68. Please Help !