I’ve had this same problem, no idea why it happens. I gave up on my editing experiments because of it. If you figure it out let me know.
edit:
though now that I think about it, I read some where that it might be either what program you use to convert file types or what file type you convert to. Are you using MP4?I am working with AVI files, and even before I render them they’re screwing up.
I don’t really know how to help you without basically sitting right next to you and watching your process through to see where you’re going wrong.
DO NOT INVITE THIS MAN TO YOUR HOUSE