Why do my clips jitter after exporting in iMovie?

When i preview my videos in iMovie they are perfect quality, smooth, clear with no problems. But when i export them they jitter and miss out frames, making them almost unwatchable. I have tried different export settings, frame rate ect. Please help!

Ps. the footage I am editing is Call of Duty footage, recorded with an HDPVR at 59.95 fps.