Thanks for your reply.
I realise it says "[VD_FFMPEG] XVMC-accelerated MPEG-2." in the attachment ffh264vpdau.txt, but it is in fact an H264 file. In fact, when I run mplayer The\ Simpsons.mpg, in the output is
Opening video decoder: [ffmpeg] FFmpeg's libavcodec codec family
Selected video codec: [ffh264] vfm: ffmpeg (FFmpeg H.264)
As I mentioned, when using the onboard video, it is able to play the file. I'll attach equivalent outputs soon.