I am having the same issues all of a sudden. I’ve been using FCPX for a few months without a problem and all of a sudden i keep getting black frames in-between clips. It happens if i move clips around within the timeline and it makes no difference weather the magnetic timeline is activated or not. The frames cannot be deleted all of the time either which makes it especially frustrating. I also have some random single frames from clips that will pop up in-between regular clips.
Another issue is that the ‘E’ and ‘W’ shortcuts for inserting clips dont always work. IE the ‘E’ will place the new clip in second last place in the timeline instead of at the end and the ‘W’ wont work at all.