rendering slow and getting dropped frames in timeline --> Error trying to share

After I drag footage in my timeline, it renders very slowly and then plays back with dropped frames intermittently dispursed throughout, causing black screen flashes. My 2016 Imac is plenty powerful with 16gb RAM, and I have tried creating new projects with various settings from 1080p 24p, to 1080p 30p, to 4k 60p. They all have the same result. And the white dots that signify rendering are barely disappearing as fast as they used to. My footage involves a green screen that I am applying the keyer mask to, but even before I apply it - same result in dropped frames. Any ideas?

The error code I get when trying to export my video is:

Video rendering error: 10008 (Final Cut Pro error 10008: renderVideoFrame failed)

Thanks,

Jorge