I'm having a similar issue, except I'm using Plex running on a MacBook Pro. All of my movies are m4v AAC h.264, all within the realm of files that should direct play from my computer to the Plex channel on Roku, but lately they will stop mid-stream and then pick back up, only this time they're being transcoded instead of played directly.
I am having the same issue also since 7.2 was released. I am running minidlna server. A movie will stop playing and throw the error "File Not Playable" in the same exact spot. The only way to get past this is to take note of the exact spot the video fails and then restart the movie from the beginning and fast forward 1 or 2 minutes past the point of failure.