Glitches on iMovie

When I’ve been editing using Imovie I’ve noticed that my video clips will glitch from time to time. It only happens when I play the entire full movie, not the individual clip. It also will sometimes glitch with previous video clips entangled. Even when I export the the movie it will contain these glitches. This has happened since I got my Mac and I’ve had it for a year. Does anyone know any solutions?