Barry,
I’ve had this trouble intermittantly and in my case it seems to be caused by odd render files getting into the FCP package contents. I’ve no idea how this happens (scratch, render etc. files are always set correctly as far as I’m aware), but it does seem to be an issue.
Anyway, have a look in Applications, then CTRL Click Final Cut Pro /Show Package Contents / Contents / at this point have a look to see if there are render files or a render folder. They can sometimes appear inside the MacOS folder.
This has solved the same issue for me a number of times. Hope this helps.