it doesn't really matter which content, take any movie, it does it with all of them......funny thing is, it usually starts out okay, and then by maybe a 1/2 hour into the movie that is when it starts....
another problem is that it doesn't respond when you ask it to pause, or fast/rewind....then a half a minute later it suddenly responds....the reason I mention this is because pausing the movie , going back to the info page and then starting it up again, often will get the captions back....but they eventually start going unglued again, soon after that.
What I could do is download a movie from Amazon Prime on my computer, and see if the same thing occurs, I think I will try this next and let you know how that turns out.
it's a roku 2, model 2720X....now: I just watched an entire movie while on Amazon prime on my computer and not once did the caption problem occur, so, it can't be the streaming from the original Amazon source.....
next would have to be to try what you just suggested, basically, you want me to erase the app, re-boot the Roku box and then try adding back Amazon as a channel...right?