All of my kids had this experience and they are all very different learners, readers etc. In my experience mumbling and punctuation issues are very common with the precocious readers as they want the higher interest content but may not quite there with the complexity of words, syntax etc. I will defer to a reading specialist for the appropriate intervention, but what worked for us in this situation was as follows:
1. The books they read to themselves were of their choosing and on their level
2. The books they read aloud to me were a level or two down--I found the mumbling was related to decoding harder words. This allowed for the mumbling to stop as they weren't stuck in decoding words the whole time, and rather could focus on comprehension.
3. Also during these books they had to stop at the punctuation for a count of 1 or 2 depending on the mark. Initially they spoke it out loud, then moved to silence with finger counting, then in their head. Again, the trouble with punctuation seemed more to do with decoding than anything else.
4. Mine also hated the stop to discuss the context, so I had them do a comprehension on-line program like Headsprout, Raz-kids or K5 Learning which supplemented their comprehension.
All in all this was less than an academic year intervention and it put them on the path to solid reading and comprehension. Hope this helps!
