Exactly what is INSIDE the video you are editing? Report back with the codec details of your file, use the program below Free program to get file information for PC/Mac http://mediaarea.net/en/MediaInfo/Download - when you analyze your file in MediaInfo and post a screen shot in the forum, do so in TREE view - post your information IN your message, not as an attachment that someone would have to download
If I were to make a vote, I'd go with "your video is VFR". As John suggested, transcode to CFR with Handbrake. Likely it'll all come back togethe nicely 🙂 That said, if you are going to post, put LOTS of information in your post or we'll all be guessing and it takes longer to solve what is essentially your problem that I'm sure you'd want to be solved faster!