Flashing Problem

Here is a very strange problem. Completed a 5 min. video for YouTube with footage from two Panasonic cameras. As I was winding up editing had several frames from both cameras that began to strobe on the screen during playback. Have latest Yosemite 10.10.4 and latest FCPX (10.2.1) on an older Mac Pro 2.8 Quad Core. Shut machine down and rebooted. Flashing was gone as I played project back. Uploaded to YouTube at 1080 and when it came up on YouTube several frames were strobing but not those strobing initially before I rebooted.

Then deleted project from YouTube. Uploaded at 720 and the video comes up on YouTube with no strobing. Haven't tried to make a blu ray at 1080, but this is sure disturbing. Have never had this problem before and am stumped. By strobing I mean those frame are flashing on and off rapidly. I did not reboot before sharing on YouTube a second time. All frames that strobed had been stabilized but why should they strobe on one playback or share to YouTube and not another.

Should have said clips, not frames. The entire clip is strobing when it happens.

More information would be helpful. What is the video codec and frame rate of the clips in your timeline? What is the frame rate of your project? Did you create a master file from FCPX to further encode for YouTube in compressor, or did you share directly to YouTube from FCPX? If you created a master file, what compressed file format did you make to upload to Youtube? how did you import your clips into FCPX? Did you optimize footage upon import?
Answers to these will help to identify your issue.

Did not change any of these settings. The preference is set for prores 422 proxy for import. No separate compression was done after completing edit. Did not ask it to create a master file. After edit completed, selected to share to YouTube as usual. Have never changed any of these settings. Only change was going from 1080 to 720 and with 720 no strobing. The only time ever see strobing in the past was when rendering wasn't done, but it was done before sent to YouTube.

Uploaded another video to YouTube today and no problems. Think it may have something to do with rendering that is not complete. Have noticed that after rendering appears to be done and you don't see anything running there sometimes are some red lines. Then if you give it a command to render all. It then completes a few more renders.

There is a possibility that you have some corrupt render files in the problematic project. I would suggest for that you select the library in FCPX and then delete the generated render files choosing ALL render files. Exporting an unrendered timeline is fine, as FCPX will automatically render upon export. You would need to render the timeline again if you are unable to playback the project smoothly in the timeline.