I've been saying the same thing for months. Mine stops 2 hours into the film (no matter how large the file is). I'm using XP and WMP11. The DSM510 stops, BUT the DSM520 does not stop. And you are correct - I've tried other media servers and they do not cause the dsm510 to stop, but for security reasons I'm stuck with wmp11. I sure wish dlink would find and SOLVE this problem. It certainly may be that WMP is partly to blame, but since other products do not exhibit this same behaviour, it does seem that it can be resolved by correcting the firmware.
@ zoulourichard
When you start a movie, would you pause it for a length of time (say 15 minutes), then see if the film will stop 15 minutes earlier? That's how it is with mine. I would say if nothing else, that proves several things (#1) movies are encoded correctly, and (#2) it's not a "file size issue".
-Jack