is this when recording or when playing it back? if its playback, try transphering the files to your device nativly first, like your HDD or something. as hippo mentioned, its likley a reading slow issue, could be the card reader or the card itself, if its still screwie after youve transphered it try the same on another computer or device. might be the device having a codec issue (unlikley but possable) if its screwie on both when nativcly on the device, might be your card is too slow for the gopro to seemlessly write to it.
let us know how you get on with that, might have other things to try for you
also try another program to view it on, sometimes some programs have issues with random files for not much of a reason.