All shot 24pA, edited on 24p timeline, cleaned up render files and re-rendered, exported and still the same problem: the clips on the FCP sequence timeline play fine but the exported movie picks up random shots that do not appear in the source sequence. Result: I end up with uninvited cutaways that just spring up in the movie and I have no idea how to fix this issue. Top it off with a requirement to show the film to a bunch of viewers in a couple of days so boy would I like to hear if anyone's come across and resolved this problem.