FCP X freezing during Mastering

Final Cut Pro X keeps freezing while I'm trying to create a Master File. It happened previously (last week) and at first I could copy the timeline onto a new project and it would work.

But then on the next video I made, that fix stopped working. Thinking it was some corrupt clips (because while filming on my iPhone I noticed the screen freeze occasionally - I was using a Zhiyun Smooth 4 gimball with FilmicPro app), I spent 3 days trying to make a Master File of each small section of the film until I thought I'd found all of the ones that didn't work - I cut out those sections but it still froze.

I ended up cleansing my iMac of FCP X and all of it's connected files (including all very recent footage) using AppDelete, then used Onyx to search for problems (I also checked my hardware), then re-installed FCP X and all seemed well for a few days, I made 2 videos. Today I finished editing a third and it froze again. I tried creating a new project like last time but that didn't work. Each time I tried the suggestions on the support page it froze, always on a different percentage of completeness. I've tried clearing the Preferences. However, when I went back to the previous film and created a Master File, that one worked fine.

I'm running out of ideas. Is it FCP X that's the problem or my footage? The previous film footage was made on the same equipment so I can't see how it could be the footage.

I'm not a computer expert so if you have any answers, please keep them fairly simple and give easy to follow instructions. Thank you.

P.S. My iMac is late 2013, 32Gb RAM, has plenty of storage left. OS is Mojave.