iMovie freezing on my MacBook during editing

Here are a few things you can try to isolate the issue:

Try booting up in Safe Mode and, after everything loads,

immediately reboot in normal mode. Don't try to open iMovie in Safe Mode

because it will not do so. After rebooting in normal mode, see if the freezing

issue is resolved.

If Safe Mode does not solve it, try redownloading iMovie. To update/redownload

iMovie 10, drag your iMovie 10 app from the Applications folder into the trash,

but do not empty the trash. Sign in to the app store and open your purchases

folder by clicking on your name at the bottom of the app store side bar. You

will see your purchases displayed, including iMovie 10. Redownload/update

iMovie from there. If not successful, drag your present iMovie app out of the trash

and return it to the Applications folder. 

If still not working, and if you haven't already tested with

another project, try creating a new project and see if you can share out with

different media and also with the same media that you are working with

now. 

Another thing that you can try is to create a new user account

(System Preferences/Users and Groups) and see if iMovie will work properly in

the new user account. 

-- Rich