fps changes during the record I recorded myself using my LG G3 smartphone. Now I see that one video is 28.97 fps, another video is 28.81 fps according to the QuickTime Player stats.
When I edit the videos in Premiere, the sound sometimes don't match to the video and when I sync the audio and the video, it loses the sync again after a while.

Is it possible that the fps is changing during the record?
It is possible that the device is dropping frames when recording. If there are a lot of apps running or the device is running low on memory. I would try rebooting the phone then try to record video and see if you experience the same problem.