This has been reported a few times, and the cause of playback distortion has been traced to the player or computer. The small addition of subtitle data can push a marginal computer beyond its ability to smoothly play the video (even if video played fine before using Alti-Force).
In VLC media player, try slowing the playback speed to half or slower. In most cases, the video will play smooth again, indicating the video with Alti-Force was recorded without error. In other cases, memory cards have been the culprit, especially the 64GB sizes. View the Compatibility tab on our sensor pack details page for more reference information and a link to an article worth checking out. Formatting the memory card in the camera is a possible corrective action, as well as good periodic maintenance.