Video MP4 file stops and starts
Drag the clips into your editing software and go frame by frame. If the freeze doesn't occur, the playback of the video might have lagged behind, causing the freezes. If the freeze still persists when going through it frame by frame, it is likely that something about the clip is broken (perhaps the i, p or b-frames are mangled, assuming the mp4 uses h264 compression.)
If so, try to edit out the section in which it freezes and export the clip as a prores4444, which doesn't overtly compress the clip further, so you can work with the pores-file from that point on.