Hi, ashpr,
Here are a few things that you can try:
Deleting Preferences
Try opening iMovie while holding down the
Option and Command keys and selecting to delete preferences in the box that
appears. iMovie will open in a new library. Reopen your old library to get back
to your projects. Now see if the flickering is gone.
New Project
Another thing you can try is to copy your
media into a newly created project and see if that cures the problem. To do
that, do an Edit/Select All, Edit/Copy, of the media in your project and then
Edit/Paste it into the newly created project.
Safe Mode
You could also try booting up in Safe Mode (see Finder menu for instructions)
and, after everything loads, immediately shut down and reboot in normal mode.
Then see if iMovie works properly.
Redownload iMovie
All else failing, you can 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.
Hopefully one of the above will solve this.
-- Rich