MP4 Video plays in QT 7, but not in 10. Can I convert it somehow to be used in iMovie?

I went back to an iMovie project that i haven't worked on in a while and I found that a few videos that previously played fine, now seem to have broken. (In iMovie, they say "Source clip is missing" and in QT10, they say "QuickTime Player can't open...". They still play in Aperture, where i store them and they still play with QT 7, but not in iMovie or QT 10. I tried using SmartConverter to convert to another MP4 or MOV, but that didn't work. Any other ideas how I can either convert or re-encode them so I can get them working in iMovie/QT10? It is only a few videos that are having this issue, so I don't need an elegant solution so even a convoluted/hokey solution is fine.