I am having the exact same issue. Playback a movie and after a few minutes a message (Please wait while video buffers) appears. Show sometimes plays fine, but 9 times out of 10 it flips back to live TV.
I have the same issue. My network is FIOS Gigabit and the receiver is hard-wired so I know it isn't a bandwidth issue. I think this is a software issue, but since I didn't write the code I can only guess. It is very frustrating and repeated calls to DirecTV support has not resolved the issue.
I had same problem but after you push the replay pause the movie and wait till the orange buffer bar gets about half way then you can watch your movie. What happens is the replay goes faster then the buffer so it catches up in about 5 minutes.