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I remember being frustrated by this problem when I had to read out loud in first and second grade. It drove me nuts. I also didn't have this problem when I read silently --- I feel confident of this statement because I was quite aware of it when reading out loud. It was like my mouth couldn't keep up with my brain.

This is excatly what my DS said when I asked him what was going on. He read with beautiful expression, so was obviously scanning ahead. I think he kept losing his place and his eyes were jumping all over the page. Again, he did this less when reading books at the outer limits of his skill. I wonder, OP, does this problem vary with the difficulty level of the book, and how? DS's odd pattern (doing it more with easy books!) reassured me that it was a weird gifted thing.

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he wants to get the expression just right and often re-reads a sentence until he likes how it sounds.

That's very interesting. I have never known a child who did this. Most kids I know read basically in a monotone till they progress to fluency and then expression jumps. I feel like I can always tell an older child who is struggling with reading by the robotic read-aloud voice.