Although this thread is a few months old, I have to add that I have similar problems with new hardware (FX Go5650 in Dell Inspiron 8600) and the newest driver (5328). I keep watching the movie and at some point mplayer only shows a blue window (or fullscreen). Even switching from xv to gl does not help then, while restarting X does. It's annoying.
Seems to help to move the mouse around every now and then so that anything like dpms timeouts or acpi events on inactivity are prevented. Not sure about that yet.
NVidia, any idea?