Problems with .mov files imported from Sony video cam

Hi
I have a Mac 2,8 GHz i5 with 8GB, and I have a nice Plex Media Server installed on it and connected to a Samsung SUHD.
Everything is perfect for files with format .avi, .mp4, etc, but for some reason my home videos recorded with a Sony handycam some years ago and imported to my Mac in a format .mov are not working very well. They start playing but every 5 or 6 seconds the video is stopped.
Any idea would be welcome.
Regards, Borja