Sounds like your computer is too slow. Playing back mpeg4 needs a lot of horsepower in HD. You won't be able to watch it on a crappy netbook (like the one I'm typing on now). What are your computers specs? It is unlikely that the footage is recorded that way. If the memory card is defective or not fast enough the camera usually just stops recording. Try setting the GoPro to the lowest resolution WVGA 848x480 and copy that to your 'puter. that would be the easiest to playback.