Since you are working on a feature I’m going to assume you have a rather long sequence or sequences. You may want to try:
1. Having ONLY one sequence open at a time.
2. Sub-divide that one sequence into smaller sequences. ie show the producer/director 10 minutes at a time.
3.Try saving the smaller sequences as their own projects to help cut down on the number of clips in it.
I have run into times when it seemed FCP just had trouble keeping up with lots of open clips.
Again since everything was stable and just now is showing trouble I would look to see how full your media drives are. If you are over 75% you may be having trouble.
Hope this helps.