I have the exact same problem. Downloaded from my fav torrent site and none of the new mp4 files work. I changed a few settings and now they won't play at all.
Inititally, the non-encoded file played perfect audio with a black screen and the various encoded files either did the same or they stuttered as if the bandwidth was low. Now, I've tried a few things (tried forcing encoding of mp4 and upgrading pms from 1.82 from 1.72), and all i get is a playstation error code.
If I play older files either mp4 or avi etc, they all work.
Anyone have any ideas??