Events in imovie 10.0.5 become blurry

Hi,

Since a while ago I have a problem using iMovie. Some events in iMovie get extremely blurry and wont play smooth (they used to look good). When looking at the original files in Finder they are still ok. How can I fix this? I have a lot of projects that have been destroyed in the same way and it would be very much work to import the movie clips again and replace the blurry ones. iMovie version: 10.0.5

Best regards

Farid