I don't believe you can downgrade to 3.1.3 any more. When you restore an OS image, iTunes checks whether it is a valid image with Apple's servers. After a maj

I don't believe you can downgrade to 3.1.3 any more. When you restore an OS image, iTunes checks whether it is a valid image with Apple's servers. After a major version has been out for a while, Apple stop approving installs of older versions.