Replying to GoPro help

Hot lapped Breck this past Saturday and got some pretty good footage but when I imported the clips into iMovie they were super glitchy. They start and stop and skip over parts of the vid, ex. skipped a whole kink on the 5 kink. Seems like it's only in iMovie though because I sent a vid to my phone and it played perfectly. Any tips/pointers as to what's wrong would be appreciated. Also, it's a hero 3 plus black edition, if that makes any difference.