Clip thumbnail images won't load.

I have had this recurring problem where the thumbnail image for clips, in the timeline, and the event browser won't load. So in order to see what the clip is I have to scrub over it, and this takes up an enormous amount of time searching for clips by scrubbing rather than just seeing the thumbnail image. I used to fix the problem by quitting, and by the way the program hangs, says its canceling background tasks forever, and I have to force quit. I have run disk utility on my HD, I have verified and repaired the external where all my video is located. I've closed the library and reopened it. Restarted the computer. I haven't updated to the latest Mac OS yet, or to the latest FCPX (I'm running FCPX 10.1.4 and OS 10.10.1), because I am in the middle of about 10 different projects and would rather not upgrade until they are all done. It was a glitch I was tolerating because I could usually fix it by quitting and restarting, but now they just won't load at all anymore. Any thoughts?