I have a six year old who does the same thing. I think this is why it's good to still to have kids read aloud, even if they are independently reading at a high level.
I often notice my son paraphrasing what he is reading, which leads me to think that he is reading ahead silently and his speaking isn't keeping up with his reading. He often substitutes words or makes silly mistakes like you described.
I've read that it destroys the flow/pleasure if you keep stopping the child. I wait and see if my son self-corrects. Most of the time he does once he gets to the end of the sentence and notices that it doesn't sound right. Where he doesn't, I silently put my finger under the word that he got wrong and he comes back and corrects himself.
I sometimes wonder if this skipping and paraphrasing causes my son's teachers to assign him to a lower reading level. While the books they give to him for independent reading are almost two years ahead of grade level, they are still far below the books that he reads independently at home. Have you run into this?
