This happens when your video clips are very large in size and your trying to play the video clips directly off your camera device or your memory card. 

A USB port cannot handle the bandwidth load of a large video clip file during playback. 

Simply copy the video clip from your camera device or memory card to your computer's local hard drive. Then play the video clips from your computer's local hard drive disk.

The USB port is designed to transfer video clips not for real-time playback.