Issues seeing video in the viewer

When I am working on my project in fcpx the video in the viewer keeps disappearing. If I play my project I can still hear the audio but there is no image in the viewer. Is this a bug? Something I have pressed? I have no idea.

Sometimes when I restart my computer the everything magically works perfectly ... and then things will not work so perfectly. I am unable to scrub through the video and see anything, the audio is still there. However when I scrub through with the blade tool selected I can see the video.

It's only a 3 minute project, with converted .mov files from a Sony Ex 1, GoPro, a small Canon Video camera that shoots 1080p.