Videos in a slideshow don't respect synchro when exported

I’ve this strange problem in my slideshows, assembled with photo (jpg) and video (MP4), when I try to export it:

  • No problem exporting to the “standalone” version
  • exporting as .mov, the video doesn’t respect the transition with the wished smooth dissolve between video and photo or video and video, but the video stops for a while with an unpleasant still image before continue with the following file.
  • If I convert the MP4 in MOV I can resolve the problem, but the original MP4 of the camera is really too much better than the mov I got with “MP4-converter” App.

Could you help me, please?