THE MILLION DOLLAR QUESTION: Projects keep becoming corrupted

So I keep importing my mp4 videos into iMovie (obtained by using the Activity Window method to download clips online, then converted it to mp4 using Handbrake) but every time I import a video (it is workable at first) and turn on my computer next time to start working on it, the clips become unusable. There is no playback of any sort in the playback window, no audio or picture, and while occasionally the thumbnails will be appropriate (still no audio though) they can't be used. Then other times the thumbnails turn into these gray and black things that only display a random thumbnail picture (not necessarily the proper corresponding one) when it is scrolled over. I think the videos are getting CORRUPTED but they work when played with quicktime or on my desktop and when they are initiaaly imported into iMovie. It's getting impossible to make any headway on a project when the clips just won't work. MY QUESTION: How do I prevent these corruptions from happening or once I trimmed and edited an imported video that later goes corrupt, how do I un-corrupt my carefully edited work?