My neutered male cat is constantly bullying my spayed female cat. What's wrong and how can I stop it?

My male cat is about 9 months old and he's a Maine coon. We had him fixed a few months ago. He isn't de-clawed though. We got our female cat about 6 months after we got the male. She is a small black cat. She is both spayed and de-clawed. They seem to get along 50% of the time, washing each other and playing nicely. The other 50% of the time however, my male cat is hunting her down, pouncing on her, biting her pretty hard and keeping her pinned down with his teeth and claws. She cries and tries to run away but just is not match for him. Is there a reason he's doing this? And any ideas on how to stop it?