Problem with a video after it has rendered...?

Hello!

I'm having a problem with this video I'm trying to render. The render process itself appears to work great, but when I review the footage, the video will freeze at a random point, then the audio will continue for a few seconds, then it will stop too. It will be in this frozen state for the rest of the video. The video is in 4K and is almost 40 minutes long. I've tried multiple fixes such as cutting out the section of video it freezes on - I've tried exporting at different qualities - I've tried deleting the rendered files and all that - I've tried restarting my Mac - I've tried restarting Final Cut, however none of these have worked. 

I'm on a MacBook Pro. I'm not a technical person so I'm not really sure what information to provide. In the video I do use some phone footage as well as the DSLR footage, and I was wondering if that could be the problem. The video plays fine in Final Cut. As you can imagine it's a rather frustrating problem. I'm hoping someone with a bit more technical know how cold suggest a few more fixes as I'm stumped...

You say that the video plays fine in FCP. Before that, you said that when you "review the footage..." Is this playing back the exported video?

How full is your internal disk? How about any external disks?

Which version of FCP? macOS version? Amount of RAM in your system? Exact year and model of your MacBook Pro? If you exported the video, which codec and settings did you choose? Thanks.