These are results of minor transmission errors. When letters are missing, it means that your decoder chip didn't see a frame of video. Usually, if letters are missing, they will be missing in sets of two. Why? Because two characters are sent on each frame of video. Now, this isn't usually a problem with the decoder chip, but rather a little glitch in the video signal.
When you see a white box, that means that there is a major glitch in the video signal. The captions have become too corrupted for your decoder to sort them out. This occurs when the caption information is interrupted--perhaps by a bad edit, perhaps by a lightning strike, perhaps by the broadcaster digitizing the signal.