Hi,
First message here, excuse my mistakes please.
I had this same problem and after the solutions here listed were far beyond my grasping/didn’t work for me, I finally found a solution (for my case at least) at the Adobe Community.
In my case, the problem was that the video was recorded with a variable frame rate. I used Hadbrake to convert it to a constant frame rate and the problem disappeared after I substituted the original clips with the new versions.