My text-to-speech function is skipping. If it's reading a large block of text, it begins skipping around the 4 or 5 hundred word mark. What gives?

My text-to-speech function is skipping. If it's reading a large block of text, it begins skipping around the 4 or 5 hundred word mark.

It's been happening for a few months, but I didn't always have this issue.

Help, please!

(Details about my computer:

MacBook Pro

Retina, Mid 2012

Processor 2.3 GHz Intel Core i7

Memory 8 GB 1600 MHz DDR3

Graphics Intel HD Graphics 4000 512 MB

Software OS X 10.8.2 (12C2034)

)