OK, it looks like I found the source of intermittent problems with video playback - /usr/libexec/gst-video-thumbnailerd

It can be stack for some reason and if I kill it I am able to have a stable video playback ... well, in KMPlayer. Because system media player starts this daemon automatically if you start media player.

EDIT: I filled a bug https://bugs.maemo.org/show_bug.cgi?id=8165 you can vote

But no more annoying "unsupported codec" even for clips from backside camera.