1- Library and media should not be on the system drive, but on a very fast external drive.
2- What percentage of your system drive's total capacity is free space?
3- Run Disk Utility to Repair all connected drives, especially the system drive.
4- Hold the Option and Command keys together while launching FCPX, when prompted, delete the preference files.
Thanks Fcpx guru, I have done all that already.
Also I send MacBook to apple care , they checked they saw no hardware problem.
They recommended just to reinstall a clean macOS.
I done that and for a little time was everything ok. Yesterday the symptom returned.
Um, did you install this AFTER 12 noon, on a Tuesday or Thursday, when it is not raining? Cause that could be the issue. Or maybe check the air pressure in the tires?
Yeah, what Ronny said.
* But if you reinstalled OS X from scratch, did you restore from a backup (TimeMachine or something) or did you install everything from scratch? If you restored from a backup, we could be restoring corrupted FCPX system file(s).
* In addition to Ronny's testing the user account, I wonder if we have something corrupted inside of a Library? That's happened to me twice in the past 7 months. So could this be Library specific? A corrupted Library would not be effected by re-installing anything, also.