Exported film shows a black screen at random points in the film.

I have exported the film using h264, full HD etc as I usually do (Never had this issue before) the exported video is successful but then playback reveals a balck screen for a few seconds at about 4 random points in the film. The sound still plays) I'm unsure what this issue could be and have never had any issues like this before. The timeline when double checked showed the footage plays fine and the frames are present and not toggled to bl;ind etc on any given line. Any suggestions whast might be caiusing this issue>?