Movies won't play in full

In trying to watch (streaming from primewire, usually) 2 hour long movies, the playback stops before the end of the film (3/4 through, maybe) and then it's no longer available to watch, as I can't forward to the same spot (have to replay).
I have no problem watching anything 1 hour or less.
Transcoder settings are default; although I don't see a temp folder.
any help is appreciated, as I'm baffled by Plex, generally..