Just to be clear, the problem occurs when you playback the movie on the camera itself? If so, sorry, no idea what would cause that. But I wanted to make sure since you didn't specify and there are a bunch of reasons this could happen when playing back on other devices.
I recently just started getting this problem. When shooting a movie, all goes well and normal while shooting. The problem comes in when I playback the movie...it is in super hi speed motion. I must have selected a setting by mistake, but I can't correct it.