I was having trouble exporting a project in Motion 4, and getting a clean render. For reasons I still don't know, I would get a few frames with a piece missing, randomly. Each render was nearly two hours long, I was coming up on deadline, and all the render had a few random bad frames each time.
Desperate, I stumbled across a work-around: I set the render controls to output stills instead, and every still frame came out perfect, in just a few minutes. I brought the .tif file stills to the FCP timeline and turned them into a rendered stand-alone movie, and it solved my issue.