You can find this in the text animation section. Click the down arrow next to your text in the time line. On the right side you will see Animate: > go ahead and hit that drop down. Check opacity. Key frame your start 0%-100%, and then turn the randomizer on. I'm doing that from memory, so there may be a little give and take. But that should at least get you in the right direction. Try playing with the opacity level too.