Choppy "Projects" in iMovie
I'm running Mac OS Mojave 10.14.6. I have downloaded many video clips from my Sony HDR-SR11 video camera with good results. I have then dragged the "clips" from the "Media" to create "Projects". All worked well. However, for quite a while now, when I view any "Project", the video is "choppy". However, when I run a viewing of the "Media" from which the "Project" was made, the video is fine and runs smoothly. I have called Apple support numerous times and been given a number of possibilities to try to correct the problem. To date, none have worked. I want to download the latest OS (Big Sur?) but I'm afraid I might inadvertently damage or eliminate my iMovie videos. Additionally, I want to make "Projects" of all my videos and put them on flash drives and/or DVD's. I then want to put the completed videos in the "cloud" so I can view them on my iPad without having to bring flash drives or other external devices when I visit friends. Most of my videos are personal family videos of international visits with my son. They are extremely important to me and I want to be able to make copies of all for safety. I'm 76 years old. In the absence of an 8 year old child to guide me through this technological maze, I'm struggling. Any advice would be greatly appreciated...
iMac 27″, macOS 10.14