This might be totally unrelated. But my older brother does this. He started when he was younger. It wasn't a speech problem as a confidence self esteem issue. To this day, he gets nervous and starts stammering on certain words and mummbling. Then it gets worse because he's embarrased by the stammering. He does it to me sometimes still and I'm his sister. Honestly I feel so bad for him. His ex wife didn't make it any better, but that's a post for a different forum! lol Not sure if this is the case for your dck, but it's just a thought

Reply