Hi,
What is the resolution, frame rate, and format of the GoPro clips? iMovie does not support 260k.
Meanwhile, you can try deleting/resetting preferences and see if that cures it. To delete/reset preferences open iMovie while holding down the Option and Command keys and select to delete preferences in the box that appears. iMovie will open in a new empty library. If so, reopen your old library to get back to your projects. Possibly you may need to reboot to set the changes. Deleting preferences is a safe procedure that will not cause data loss or disruption to your project.
-- Rich